[unix(4) cleanup 6/8] Simplify connection peer locking.

[markj]

Summary

unp_pcb_owned_lock2() has some sharp edges and forces callers to deal
with a bunch of cases. Simplify it:

- Rename to unp_pcb_lock_peer().
- Return the connected peer instead of forcing callers to load it
beforehand.
- Handle self-connected sockets.
- In unp_connectat(), just lock the accept socket directly. I believe
it is safe to do so.
- Get rid of connect_internal(). Callers verify that the socket is
connected afterward, and unp_connect() will be modified in a follow-up
diff to verify that the socket is not already connected.
- (Ab)use UNP_CONNECTING to guard against concurrent creation of
connections while relocking PCBs.

[markj]

Use unp_pcb_unlock_pair().

[markj]

sys/kern/uipc_usrreq.c

399 ↗

(On Diff #76551)

This is too simplistic. We need to use a separate counter here, and unp_connectat() should sleep if threads are waiting to acquire a lock.

[markj]

• Fix the mechanism used to block new connections when another thread is attempting to lock both sides of a connection.
• Subsume patch 8, which is closely related to this one.