tests/unix_passfd: test that control mixed with data creates records

If socket has data interleaved with control it would never allow to read
two pieces of data, neither two pieces of control with one recvmsg(2). In
other words, presence of control makes a SOCK_STREAM socket behave like
SOCK_SEQPACKET, where control marks the records. This is not a documented
or specified behavior, but this is how it worked always for BSD sockets.
If you look closer at it, this actually makes a lot of sense, as if it
were the opposite both the kernel code and an application code would
become way more complex.

The change made recvfd_payload() to return received length and requires
caller to do ATF_REQUIRE() itself. This required a small change to
existing test rights_creds_payload. It also refactors a bit f28532a0f363,
pushing two identical calls out of TEST_PROTO ifdef.

Reviwed by: markj
Differential Revision: https://reviews.freebsd.org/D43724