MFC r292914,r292957:

r292914:

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)

https://reviews.freebsd.org/D689

Submitted by: markj

r292957:

Rename `recvfd` and `sendfd` variables in recvfd/sendfd functions to avoid
-Wshadow issues with gcc