[freeciv-ai] Send useless explorers home

[Per Inge Mathisen <per@xxxxxxxxxxx>]

Explorers that cannot explore anything else - send them home, or at least
to a city somewhere. Yes, I implemented this a while back, but Greg took
it out again when the explorer code was rewritten ;-)

The proper solution is of course to send the explorers off-continent -
that is the next step.

Also makes the gohome code use ferries. There may be some unforseen
side-effects from this.

  - Per

gohome1.diff
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