test-socket-util: Add tests for receive_fd_iov() and friends.

test-socket-util: Add tests for receive_fd_iov() and friends.

Test it when sending an FD without any contents, or an FD and some contents,
or only contents and no FD (using a bare send().)

Also fix the previous test which forked but was missing an _exit() at the
end of the child execution code.