We currently only return primary time zones, i.e. time zones that are
not a Link. LibJS will require knowledge of Link entries, and whether
each entry is or is not a Link.
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.
