unlink("/") fails with EISDIR instead of EBUSY on FreeBSD; test for that
instead

Sponsored by: EMC / Isilon Storage Division
Port t_chroot to FreeBSD

- Add missing #include sys/stat.h for mkdir(2)
- Omit the fchroot(2) tests because the support is not present on FreeBSD

Sponsored by: EMC / Isilon Storage Division