[freeciv-ai] correctly evaluate ocean dangers

[Per Inge Mathisen <per@xxxxxxxxxxx>]

Now that Greg has so lovingly granted us ocean numbers, we can correctly
check if we need to preemptively build that Coastal Fortress.

Patch attached:
 - checks for dangers in correct oceans
 - no longer considers empty cities "explore-worthy"
 - new aidata variable keeps track of which oceans are still not
   fully explored (not currently used; maybe it should be an int
   of how many tiles remain out of how many total?); similar to
   the land-based variable that does the same for land, also
   unused...
 - stop worrying twice whether we can build anti-unit-buildings;
   it hardly saves any CPU cycles, and might just spend more doing those
   checks. also plays nicer with general effects.

Not really tested yet. Comments?

I thought about adding ocean check to goto_is_sane(), but problem is we
might have a canal in a city. So I thought about making an aidata matrix
of canals that exist, but then I wondered if that was really worth the
trouble...

  - Per

oceandanger1.diff
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