Simplify unp_disconnect() callers.

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; this lets us remove several refcount manipulations. Tighten
assertions.

Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26297