tzcode: Use -00 only for invalid time zones

As of tzcode 2025a, if we are unable to load a time zone, we set tzname
to "-00" to indicate an error.  This penalizes users who simply don't
set TZ or create /etc/localtime as a faster way of setting the time zone
to UTC (pointing /etc/localtime at /usr/share/zoneinfo/UTC forces us to
parse it every time for no real benefit).  To rectify this, use "-00"
only if TZ was set or zoneinit() returned something else than ENOENT.

MFC after:	3 days
Fixes:		967a49a ("Update tzcode to 2025b")
Reviewed by:	philip
Differential Revision:	https://reviews.freebsd.org/D52680

Author: dag-erling

contrib/tzcode/localtime.c

@@ -1649,9 +1649,13 @@ tzset_unlocked_name(char const *name)
     lclptr = sp = malloc(sizeof *lclptr);
 # endif
   if (sp) {
-    if (zoneinit(sp, name, TZLOAD_FROMENV | TZLOAD_TZSTRING) != 0) {
+    int err = zoneinit(sp, name, TZLOAD_FROMENV | TZLOAD_TZSTRING);
+    if (err != 0) {
       zoneinit(sp, "", 0);
-      strcpy(sp->chars, UNSPEC);
+      /* Abbreviate with "-00" if there was an error.
+	 Do not treat a missing TZDEFAULT file as an error.  */
+      if (name || err != ENOENT)
+	strcpy(sp->chars, UNSPEC);
     }
     if (0 < lcl)
       strcpy(lcl_TZname, name);