libkldelf: add a private library for kernel/kld-related ELF parsing

The libkldelf library was originally a part of kldxref(8). It exposed
ELF parsing helpers specialized in parsing KLDs and the kernel
executable. The library can be used to read metadata such as linker_set,
mod_depend, mod_version and PNP match info, and raw data from the ELF.

To promote the reuse of the facilities the ELF parsing code is separated
from kldxref(8) into a new private library.

kldxref(8) is modified to link against the libkldelf library.

Sponsored by:	Juniper Networks, Inc.
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D46719

share/mk/src.libnames.mk

@@ -27,6 +27,7 @@ _PRIVATELIBS=	\
 		gtest_main \
 		heimipcc \
 		heimipcs \
+		kldelf \
 		ldns \
 		sqlite3 \
 		ssh \
@@ -312,6 +313,7 @@ _DP_bsnmp=	crypto
 .endif
 _DP_geom=	bsdxml sbuf
 _DP_cam=	sbuf
+_DP_kldelf=	elf
 _DP_kvm=	elf
 _DP_casper=	nv
 _DP_cap_dns=	nv