Various mouse tweaks and improvements

Player-controlled mice spawn next to a vent by default, spawn at MOB_LAYER, can nibble food to regain health, can pass through plastic flaps, and can climb into disposal units.

Mouse-attacking message changed from "splats the" to "stamps on the".

A side-effect of allowing mice to eat and climb into disposal units is that they can also pull objects and click+drag things (though clicking and dragging objects into a disposal unit did not work when tested).