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/Libraries/LibTimeZone/TimeZone.h

@@ -31,6 +31,7 @@ struct NamedOffset : public Offset {
     String name;
 };
 
+StringView system_time_zone();
 StringView current_time_zone();
 ErrorOr<void> change_time_zone(StringView time_zone);
 Span<StringView const> all_time_zones();