icaltzutil_fetch_timezone now creates VTIMEZONE components using
RRULEs rather than expanding out every year (if
USE_INTEROPERATIVE_VTIMEZONES was off) or using a single time zone
back to 1970 (if it was one).

This creates more accurate time zones that are compatible with dates
whose time zones have since transitioned to a different offset (such
as a date in America/New_York in 1980).  Removed
icaltzutil_set_exact_vtimezones_support and
icaltzutil_get_exact_vtimezones_support.  Removed
USE_INTEROPERABLE_VTIMEZONES.