libc: expose execvpe for Linux compat

We already implemented execvpe internally with an _ prefix in libc so
go ahead and expose it for compatibility with Linux.

This reverts c605eea952146348e5e1ad5cab6c127d7a1bd164.

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