physics: fix and refactor unsticking

physics: fix and refactor unsticking

Closes https://github.com/DarkPlacesEngine/darkplaces/issues/165

Bugs came from 270f020888c5a03b98cc7752957243e6123c1132 trying to use
NudgeOutOfSolid on Q1BSP which currently isn't possible, combined with
a map bug on alk_caustic, and unsticking unfortunately being an
essential part of Q1BSP player physics.

Adjusts cvar defaults to match Quake behaviour.
Improves documentation.

Merges SV_UnstickEntity() and SV_CheckStuck().

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>