elfcopy: set ELF OS/ABI field when converting from binary

PR: 228934
Submitted by: Tiger Gao <tig@freebsdfoundation.org>
Reviewed by: markj, jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23504

libelftc: Consistently use size_t for string table offsets and sizes.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20702