(grow_islands, set_coastal, update_coastal_flag, coast_sea_to_land)
(coast_land_to_sea): Determine sct_coastal flag when the sector type
is set. Old method only updated the sct_coastal flag when doing
cens() or a dump(). Other uses of sct_coastal could get stale values,
including selector coastal. Rework set_coastal() into NEW functions
update_coastal_flag(), coast_sea_to_land(), coast_land_to_sea() and
set_coastal(). Fix a bug where the deity designation would not
properly set the coastal flag. Fix a bug where the one sector lake
would not properly compute coastal flag. Fix a bug that makes
fairland treat the last sector it adds to an island as non-coastal.
