MFC r354694, r354699

r354694:
elf_aux_info: Add support for AT_EXECPATH.

Reviewed by: emaste, sef
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D22353

r354699:
Improve the description of AT_EXECPATH availability.

Reported by: kib
Sponsored by: DARPA, AFRL