Improve unix socket PCB refcounting.

• Use refcount_init().
• Define an INVARIANTS-only zone destructor to assert that various bits of PCB state aren't left dangling.
• Annotate unp_pcb_rele() with __result_use_check.
• Simplify control flow.

Reviewed by: glebius, kevans, kib
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26295