MFC r273236:
Use pmc_destroy_pmc_descriptor() to actually free the pmc, which is
consistent with pmc_destroy_owner_descriptor(). Also be sure to destroy
PMCs if a process exits or execs without explicitly releasing them.
MFC r273342:
Fix a typo from r189544, which replaced unp_global_rwlock with unp_list_lock
and unp_link_rwlock.