Fix NO_SENDMAIL knob. When FreeBSD's old BSD version of vacation was

replaced with the new version in sendmail's distribution, vacation and
the necessary libraries (libsmdb and libsmutil) were changed so they
were always compiled.  This broke people who didn't checkout
src/contrib/sendmail/.  I don't know if it's best to think of NO_SENDMAIL
as no sendmail sources available or no sendmail binary.  It is now the former.

Also, remove the sendmail chapter from System Managers Manual (SMM) if
NO_SENDMAIL is defined (for similar reasons -- source not available).

PR:		31863, 31865
Submitted by:	matusita, Joe Kelsey <joe@zircon.seattle.wa.us>
MFC after:	3 days

lib/Makefile

@@ -27,7 +27,7 @@ SUBDIR=	${_csu} libcom_err libcrypt libkvm msun libmd \
 	libedit libfetch libform libftpio ${_libgnumalloc} ${_libio} libipsec \
 	libipx libisc libmenu ${_libmp} ${_libncp} \
 	libnetgraph libopie libpam libpanel libpcap \
-	${_libresolv} libsmdb libsmutil \
+	${_libresolv} ${_libsmdb} ${_libsmutil} \
 	libstand ${_libtelnet} libusb ${_libvgl} libwrap libxpg4 liby libz
 
 .if exists(${.CURDIR}/csu/${MACHINE_ARCH}-${OBJFORMAT})
@@ -51,6 +51,11 @@ _libc_r=	libc_r
 _libbind=	libbind
 .endif
 
+.if !defined(NO_SENDMAIL)
+_libsmdb=	libsmdb
+_libsmutil=	libsmutil
+.endif
+
 .if ${MACHINE_ARCH} == "i386"
 _compat=	compat
 _libncp=	libncp