Add a `jumble' key (`J') to Net, which scrambles the positions of all unlocked

Add a `jumble' key (`J') to Net, which scrambles the positions of all unlocked
tiles randomly. (Rachel asked for this; it's been being tested for a good few
months now, and Simon didn't care either way, so in it goes :)

As part of this, the front end can now be asked to provide a random random
seed (IYSWIM).

[originally from svn r5019]