Integrate tools/regression/sockets/unix_passfd into the FreeBSD test
suite as tests/sys/kern/unix_passfd_test

• Convert testcases to ATF
• Fix an alignment issues
• Mark rights_creds_payload(..) as an expected failure (see PR # 181741)

Based [in part] on the following Differential Revision:
https://reviews.freebsd.org/D689

MFC after: 1 week
Submitted by: markj
Sponsored by: EMC / Isilon Storage Division