Add option to clear caller-used registers on function return.

The WITH_ZEROREGS option for src.conf will zero caller-used register
contents just before returning from a function, ensuring that
temporary values are not leaked beyond the function boundary. This
means that register contents are less likely to be available for side
channel attacks and information exposures.

It reduces all except 1 of the simple "write-what-where" ROP gadgets in
/lib:
    grep "Gadget found" /tmp/before_lib* | wc -l
     197
    grep "Gadget found" /tmp/after_lib* | wc -l
       1
    grep "Gadget found" /tmp/after_lib*
    /tmp/after_libbsdxml.so.4.txt:  [+] Gadget found: 0x1b3f1 mov qword ptr [rdi], rcx ; pop rbp ; ret

To reproduce:
    for lib in *.so.*; do
        echo $lib:
        ROPgadget --ropchain --binary /tmp/be_mount.Sx87/lib/$lib | sed -n '/Step 1/,/Step 2/p' >! /tmp/before_$lib.txt
        ROPgadget --ropchain --binary $lib | sed -n '/Step 1/,/Step 2/p' >!  /tmp/after_$lib.txt
    done

Additionally, in some cases this reduces the number of all ROP gadgets
(quick check with /libs only):
libalias.so.7: reduction 10.000%
libavl.so.2: reduction 13.900%
libbsdxml.so.4: reduction 37.500%
libc.so.7: reduction 10.000%
libc++.so.1: reduction 14.800%
libcam.so.7: reduction 50.700%
libcap_netdb.so.1: reduction 5.800%
libcasper.so.1: reduction 14.600%
libcrypto.so.30: reduction 7.500%
libdtrace.so.2: reduction 3.900%
libelf.so.2: reduction 15.800%
libgcc_s.so.1: reduction 32.700%
libibverbs.so.1: reduction 5.300%
libicp.so.3: reduction 2.100%
libipt.so.0: reduction 28.200%
libirdma.so.1: reduction 1.600%
libkiconv.so.4: reduction 0%
libm.so.5: reduction 21.900%
libmd.so.6: reduction 0%
libmd.so.7: reduction 3.100%
libncursesw.so.9: reduction 11.200%
libnvpair.so.2: reduction 40.200%
libpcap.so.8: reduction 11.400%
libpjdlog.so.0: reduction 27.400%
libsbuf.so.6: reduction 2.900%
libspl.so.2: reduction 42.300%
libsys.so.7: reduction 2.700%
libthr.so.3: reduction 21.000%
libuutil.so.2: reduction 13.100%
libz.so.6: reduction 5.600%
libzpool.so.2: reduction 15.100%

In some cases it adds some ROP gadgets despite removing the simple ROP
gadgets:
lib80211.so.1: reduction -32.700%
libbe.so.1: reduction -22.300%
libbegemot.so.4: reduction -20.500%
libcap_dns.so.2: reduction -58.000%
libcap_fileargs.so.1: reduction -28.200%
libcap_grp.so.1: reduction -54.000%
libcap_net.so.1: reduction -28.800%
libcap_pwd.so.1: reduction -38.800%
libcap_sysctl.so.2: reduction -71.100%
libcap_syslog.so.1: reduction -15.000%
libcrypt.so.5: reduction -14.600%
libctf.so.2: reduction -.300%
libcxxrt.so.1: reduction -14.000%
libdevstat.so.7: reduction -1.600%
libedit.so.8: reduction -4.200%
libgeom.so.5: reduction -16.500%
libicp_rescue.so.3: reduction -2.300%
libipsec.so.4: reduction -31.800%
libjail.so.1: reduction -21.700%
libkvm.so.7: reduction -5.300%
libmlx5.so.1: reduction -6.300%
libmt.so.5: reduction -23.000%
libnv.so.1: reduction -.400%
librss.so.1: reduction -3.800%
librt.so.1: reduction -24.000%
libssp.so.0: reduction -21.100%
libstats.so.0: reduction -9.000%
libtinfow.so.9: reduction -3.500%
libtpool.so.2: reduction -36.500%
libufs.so.8: reduction -11.900%
libulog.so.0: reduction -67.400%
libumem.so.2: reduction -2.000%
libutil.so.9: reduction -7.200%
libxo.so.0: reduction -9.000%
libzdb.so.2: reduction -11.700%
libzfs_core.so.2: reduction -17.700%
libzfs.so.4: reduction -.300%
libzfsbootenv.so.1: reduction -26.900%
libzutil.so.2: reduction -5.600%

To reproduce:
    for lib in *.so.*; do
        echo -n $lib:
        before="$(ROPgadget --nosys --nojop --binary /tmp/be_mount.Sx87/lib/$lib | tail -n1 | cut -d : -f 2)"
        after="$(ROPgadget --nosys --nojop --binary $lib | tail -n1 | cut -d : -f 2)"
        echo " reduction" $(bc -S 3 -e "(1-${after}/${before})*100")%
    done >/tmp/reduction.txt

Most of the time the size difference is very small (<1% for >50% of the
files and >10% for only 2 files):
lib80211.so.1: size change .100%
libalias.so.7: size change 0%
libavl.so.2: size change 0%
libbe.so.1: size change .100%
libbegemot.so.4: size change .100%
libbsdxml.so.4: size change 0%
libc.so.7: size change 1.200%
libc++.so.1: size change 1.600%
libcam.so.7: size change 1.900%
libcap_dns.so.2: size change .100%
libcap_fileargs.so.1: size change .100%
libcap_grp.so.1: size change .100%
libcap_net.so.1: size change .100%
libcap_netdb.so.1: size change .100%
libcap_pwd.so.1: size change .100%
libcap_sysctl.so.2: size change .100%
libcap_syslog.so.1: size change .100%
libcasper.so.1: size change 0%
libcrypt.so.5: size change 3.900%
libcrypto.so.30: size change 1.400%
libctf.so.2: size change .100%
libcxxrt.so.1: size change .100%
libdevstat.so.7: size change 15.400%		exceptional
libdtrace.so.2: size change .600%
libedit.so.8: size change 1.800%
libelf.so.2: size change .100%
libgcc_s.so.1: size change 3.000%
libgeom.so.5: size change 0%
libibverbs.so.1: size change .100%
libicp_rescue.so.3: size change .100%
libicp.so.3: size change 1.500%
libipsec.so.4: size change .100%
libipt.so.0: size change 3.100%
libirdma.so.1: size change .100%
libjail.so.1: size change .100%
libkiconv.so.4: size change .100%
libkvm.so.7: size change .100%
libm.so.5: size change 1.700%
libmd.so.6: size change 0%
libmd.so.7: size change .100%
libmlx5.so.1: size change 0%
libmt.so.5: size change .100%
libncursesw.so.9: size change 1.900%
libnv.so.1: size change 4.300%
libnvpair.so.2: size change 4.300%
libpcap.so.8: size change 1.200%
libpjdlog.so.0: size change .100%
librss.so.1: size change .200%
librt.so.1: size change .100%
libsbuf.so.6: size change .100%
libspl.so.2: size change 0%
libssp.so.0: size change .100%
libstats.so.0: size change .100%
libsys.so.7: size change .100%
libthr.so.3: size change 2.400%
libtinfow.so.9: size change 1.600%
libtpool.so.2: size change .100%
libufs.so.8: size change .100%
libulog.so.0: size change .100%
libumem.so.2: size change 54.300%		exceptional
libutil.so.9: size change .100%
libuutil.so.2: size change .100%
libxo.so.0: size change .100%
libz.so.6: size change .100%
libzdb.so.2: size change .300%
libzfs_core.so.2: size change .100%
libzfs.so.4: size change 2.000%
libzfsbootenv.so.1: size change .100%
libzpool.so.2: size change 1.200%
libzutil.so.2: size change 0%

stand/defs.mk

@@ -11,6 +11,7 @@ FORTIFY_SOURCE=	0
 MK_CTF=		no
 MK_SSP=		no
 MK_PIE=		no
+MK_ZEROREGS=	no
 MAN=
 .if !defined(PIC)
 NO_PIC=