Now that the POSIX working group is going to require that basename(3)
and dirname(3) are thread-safe in future revisions of the standard,
there is even less of a need to provide basename_r(3). Remove this
function to prevent people from writing code that only builds on
FreeBSD and Bionic.
Removing this function seems to break exactly one port: sbruno@'s
qemu-user-static. I will send him a pull request on GitHub in a bit.
__FreeBSD_version will not be bumped, as any value from 2017 can be used
to test for the presence of a thread-safe basename(3)/dirname(3).