Unloading liquids to adjacent tiles [CR]

Resolves #2438 which requested the ability to unload liquids in adjacent tiles.  This is handy for putting out fires with water without having to step onto the tile the fire is in, among other uses.

#2438 also requested unloading X amount of liquid, but I think that is outside the scope of the code I'm committing here.  It would need to target the entire unload code, and do something similar to what multi-drop does for choosing the amount to drop.