We already implemented execvpe internally with an _ prefix in libc so
go ahead and expose it for compatibility with Linux.
This reverts c605eea952.
Bump __FreeBSD_version for the addition and add definitions to supress
compat shims in libzfs (zfs changes were merged from upstream).
PR: 275370 (request and exp-run (thanks antoine!))
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D42846