Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Bug fix: when updating headers using the gelf_update_*() functions,
the appropriate `dirty' bit needs to be set for both the Elf32 and
Elf64 case.

Obtained from: elftoolchain
MFC after: 1 month

MFP4: Add an implementation of the ELF(3) and GELF(3) API set.

Bump __FreeBSD_version.

Reviewed by: jb