Fix tsd cleanup regressions.

Fix tsd cleanup regressions that were introduced in
5460aa6f66 (Convert all tsd variables to
reside in a single tsd structure.).  These regressions were twofold:

1) tsd_tryget() should never (and need never) return NULL.  Rename it to
   tsd_fetch() and simplify all callers.
2) tsd_*_set() must only be called when tsd is in the nominal state,
   because cleanup happens during the nominal-->purgatory transition,
   and re-initialization must not happen while in the purgatory state.
   Add tsd_nominal() and use it as needed.  Note that tsd_*{p,}_get()
   can still be used as long as no re-initialization that would require
   cleanup occurs.  This means that e.g. the thread_allocated counter
   can be updated unconditionally.

src/tcache.c

@@ -263,7 +263,8 @@ tcache_get_hard(tsd_t *tsd)
 {
 
 	if (!tcache_enabled_get()) {
-		tcache_enabled_set(false); /* Memoize. */
+		if (tsd_nominal(tsd))
+			tcache_enabled_set(false); /* Memoize. */
 		return (NULL);
 	}
 	return (tcache_create(choose_arena(tsd, NULL)));