LibTimeZone+Userland: Rename current_time_zone to system_time_zone

This renames the current implementation of current_time_zone to
system_time_zone to more clearly indicate what it is. Then reimplements
current_time_zone to return whatever was set up by tzset, falling back
to UTC if something went awry, for convenience.

Userland/Utilities/timezone.cpp

@@ -32,7 +32,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     }
 
     if (time_zone.is_empty()) {
-        outln("{}", TimeZone::current_time_zone());
+        outln("{}", TimeZone::system_time_zone());
         return 0;
     }