#36 Make treaties useful again

[Markus Armbruster]

Treaties (commands offer, consider, treaty) can't
capture many important aspects of agreements commonly
made nowadays.

Agreement durations are more commonly measured in
updates or etus than in days, and `rolling' durations
(agreement stays effect until this or that time after
either side gives notice of expiry) are very common.

The list of clauses is inadequate. For instance, it
doesn't cover overflights or intrusion with spy units.

[Markus Armbruster]

Treaties have been removed from the game in commit a109de9, v4.3.33.