[unix(4) cleanup 4/8] Simplify unp_disconnect() callers.

[markj]

Summary

In all cases, PCBs are unlocked after unp_disconnect() returns. Since
unp_disconnect() may release the last PCB reference, callers may have to
bump the refcount before the call just so that they can release them
again.

Change unp_disconnect() to release PCB locks as well as connection
references. Remove code that bumps and then releases the refcount.
Tighten assertions.

[markj]

Use unp_pcb_unlock_pair().

[markj]

sys/kern/uipc_usrreq.c

2003

There is a bug here, fixed later in the series: if unp_conn == NULL we might in fact be dropping the last reference here.

[markj]

Don't use unp_pcb_rele_notlast() when we may in fact be releasing the last ref.