libkldelf: add an internal library for kernel/kld-related ELF parsing

[khng]

Summary

The libkldelf was originally part of kldxref(8). To promote the reuse of
the facilities the ELF parsing code is separated from kldxref(8) into a
new internal library.

Bump __FreeBSD_version to facilitate buildworld BOOTSTRAPPING.

Sponsored by: Juniper Networks, Inc.