Fix/refactor zone allocator integration code.

Fix zone_force_unlock() to reinitialize, rather than unlocking mutexes,
since OS X 10.12 cannot tolerate a child unlocking mutexes that were
locked by its parent.

Refactor; this was a side effect of experimenting with zone
{de,re}registration during fork(2).

include/jemalloc/internal/private_symbols.txt

@@ -383,7 +383,6 @@ psz2ind
 psz2ind_clamp
 psz2ind_impl
 psz2u
-register_zone
 rtree_child_read
 rtree_child_read_hard
 rtree_child_tryread
@@ -537,3 +536,4 @@ witness_postfork_parent
 witness_prefork
 witness_unlock
 witnesses_cleanup
+zone_register