see-always much improved

[Jim Kingdon]

I've checked in changes to help handling of see-always units.

Before, if you saved and restored a game, the coverage map would get
calculated correctly (so you could see the see-always units, e.g. when
a town in the standard game gets captured).  But merely getting enemy
information from a capture or discovering a see-always unit the usual
way wouldn't do it.

Took me a long time to figure out why my code in combat.c wasn't
working the first time - turns out I hadn't thought of the case in
which enemy information is revealed concerning the *occupant* of a
see-always unit, rather than the see-always unit itself.  With that
fixed, the code seemed to work for me (although I haven't had much
chance to play since then, to confirm).

As usual, feel free to point it out if there was a better way or some
problem with my code or whatever.