Sprinkle gate system

[Raphfrk]

This causes gates to be automatically created when new chunks are loaded for the first time.

They would link to a random gate somewhere in a bounding box. Gate names would be random

/serverport randomgates gate_type min_x min_z max_x max_z probability

I think this algorithm will give a proper distribution

• when a new chunk is loaded pick a random other chunk in the box
  -- check if there is a gate already linked to it
  -- If so, do nothing
• other if that chunk has not been generated
  -- link to it
• otherwise, do nothing