fairland: Eliminate global variable @fl_status

fairland: Eliminate global variable @fl_status

grow_one_sector() sets @fl_status when it fails to grow a continent.
grow_continents() uses @fl_status to find out whether
grow_one_sector() succeeded, and main() uses it to find out whether
grow_continents() succeeded.

Change grow_continents() to return success / failure.
grow_one_sector() already does.  Use the return values, and eliminate
@fl_status.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>