Add different physics hud _acceleration_max cvar for slick

Add different physics hud _acceleration_max cvar for slick

While on slick acceleration is much higher, so the default
_acceleration_max is useless until very high speeds (e.g. ~2800 qu/s).
Reuses some code from strafehud. Later a lot of the shared code should be
migrated to a shared file instead.
Currently physics hud only uses csqcplayer instead of fetching the
playermodel from the server. Using strafehud's code this too would also fix
the associated issues