fairland: Try harder to deliver the requested amount of land

fairland: Try harder to deliver the requested amount of land

Planned island sizes are random with an expected value that matches
the average size requested by the user.  Can be off quite a bit when
the number of islands is small.  Also, actual island size can be
smaller than planned size when space is tight.

Instead of picking random island sizes independently, pick a random
split of their requested total size.

To reduce the probability of islands not growing to their planned
size, grow large islands before smaller ones.

To compensate for inability to grow, carry the difference over to the
next island size.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>