Userland: Invoke tzset in applications that care about time zones

In most applications, we invoke tzset once at startup for now. Most of
these are short lived and don't need to know about time zone changes.

The exception is the ClockWidget in the taskbar. Here, we invoke tzset
each time we update the system time. This way, any time zone changes can
take effect immediately.

Userland/Utilities/date.cpp

@@ -29,6 +29,8 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     args_parser.add_option(print_rfc_5322, "Print date in RFC 5322 format", "rfc-5322", 'R');
     args_parser.parse(arguments);
 
+    tzset();
+
     if (set_date != nullptr) {
         auto number = String(set_date).to_uint();