(pln_mine, pln_dropoff): Split off aerial mining into new pln_mine().

(pln_mine, pln_dropoff): Split off aerial mining into new pln_mine().
Callers changed.

(drop): Do not disclose anything about target sector before planes
actually got there, unless the target sector is owned by the player or
an ally.  This plugs a major loophole.  Consequently, the command can
no longer always refuse to attempt to drop stuff where it won't work.
Instead, the planes fly out to try.  Other plane commands (para, bomb)
behave exactly the same.  Also closes #909859.
(pln_dropoff): Cope with planes trying to drop where they can't.

(drop): Refuse to drop civilians into allied sectors, for consistency
with other means to move around stuff.