MFC r292914,r292957:

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@293272 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f