tests/unix_seqpacket: provide function that returns bound & listening sock

Use the function in mk_pair_of_sockets(), and in four existing tests -
'listen_bound', 'connect', 'shutdown_send' and 'shutdown_send_sigpipe'.
While here make mk_pair_of_sockets() return pointer to sockaddr_un instead
of path. This also fixes bug of returning pointer to stack memory of
returning function. However, the only caller that cares about this return
is temporarily ifdefed out. No functional change intended.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D43852