Only provide execvpe(3) when needed

[brooksdavis]

Check for the existence of execvpe(3) and only provide the FreeBSD compat version if required.

Motivation and Context

OpenZFS currently unconditionally defines execvpe in the SPL on FreeBSD. I'd like to add it (actually just expose it) on FreeBSD.

Description

Enable an autoconf check for execvpe and check for HAVE_EXECVPE before declared if defining it.

Technically it's a library ABI change on FreeBSD, but probably not in a way that matters?

How Has This Been Tested?

Trivial and it compiles.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

Nice and straight forward. I'd only suggest adding a comment to lib/libzfs/os/freebsd/libzfs_compat.c mentioning in what version of FreeBSD execvpe() was added.

[brooksdavis]

I've added a note claiming it will be added in FreeBSD 15. I might merge it to earlier versions, but it depends how many more issues like this one I run into.