(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.
To save space, the ancients invented `variables': a collection of
key-value pairs, missing means zero value, space for `enough' keys.
This complicates the code, as assigning to a `variable' can fail for
lack of space. Over time, `enough' increased, and for quite some time
now `variables' have been *wasting* space. This changeset replaces
them, except in struct mchrstr, struct lchrstr and struct pchrstr,
where they are read-only, and will be replaced later. It is only a
first step; further cleanup is required. To simplify and minimize
this necessarily huge changeset, the new item[] arrays have an unused
slot 0, and the old variable types V_CIVIL, ... are still defined, but
must have the same values as the item types I_CIVIL, ...
