[crossfire] move_allow attribute

[Mark Wedel]

  Thinking a little little about transportation objects (boats, horses, etc) and 
came to the realization that at some level, a move_allow field is needed.

  The basic idea is that move_allow would override any move_block.  The case I'm 
thinking about here is boats - players normally can't move on the water.  While 
the move_block of the water spaces for boats in harbor could be changed so you 
could get on the boat, this doesn't work so well if you anchor your boat at some 
island.

  The code to handle this would actually be fairly trivial - basically, in the 
update position, move_block &= ~move_allow

  This also allows for some other interesting things.

  For example, one could imagine creating a 'bridge' spell that goes over water 
- basically, it just acts like a wall spell, but has move_allow WALK on it.

  Passwall (letting you put holes in walls) would be another case.

  One question would be on how to handle slow_move attributes - should those get 
blanked out also.  I'm sort of thinking yes - then you could also have a 'create 
road' spell to get through the jungle quickly.

  Thoughts? Questions?