ZTS: Get xattr tests running on FreeBSD

ZTS: Get xattr tests running on FreeBSD

This mostly involves reworking platform checks to make illumos the
exception (thanks to their unusual way of exposing xattrs). Other
platforms are able to take advantage of the recently added xattr
wrappers in libtest.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9872