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.
This changes LibTimeZone to read the current time zone from the file
/etc/timezone rather than using tzset/tzname. Instead, in a subsequent
commit, LibC's time methods will be changed to used LibTimeZone to
retrieve the system time zone.
Also add an API to set the system time zone. This method is only allowed
when running within Serenity.
This is a rather naive implementation, but serves as a first pass at
determining the GMT offset for a time zone at a particular point in
time. This implementation ignores DST (because we are not parsing any
RULE entries yet), and ignores any offset patterns of the form "Mon>4"
or "lastSun".
