Add FreeBSD 14 support

Add support to libbsd for FreeBSD 14. The work is to create a 6-freebsd-14 branch that can be released with 6.1. The 6.1 release may have more than one libbsd branch depending on the support coverage and testing between the branch bring available the level of testing.

A separate branch provides a way for users to test and migrate.

The branch is a staging point to get the main branch to tracking FreeBSD 15 or CURRENT.

The initial work is focused on the 6-freebsd-12 branch. Changes only on the main branch may need to be brought across to the 6-freebsd-14 branch.