tests: unix: add SCM_RIGHTS tests for SO_PASSRIGHTS

[kevans]

Summary

We test both the standard case where we want to reject any SCM_RIGHTS
message, as well as the case where the kernel discards the unwanted file
upon receipt.

[markj]

You might add a test case which checks whether the option is inherited by accept().

tests/sys/kern/unix_passfd_test.c

605

[kevans]

Add a test that SO_PASSRIGHTS isn't inherited from a listening socket. The new
test does stop just short of actually sending an fd over the socket, assuming
we won't mess up and return a false-positive in getsockopt(2)

[markj]

tests/sys/kern/unix_passfd_test.c

717

I'm not sure that you actually need a separate process? AFAIR connect() does not block waiting for the server to accept().

[kevans]

Simplify noinherit case- I did the dumb, unix(4) sockets won't inherently block
on connect(2).