libcommon|Cleanup|All Games: Calculating mobj movement friction

The code for calculating friction was half-commonized already, with
the function in libcommon having the logic for Doom and Heretic. This
commit makes the friction calculation fully common so that the
Mobj_Friction() in libcommon suits all games.

Removed redundant code from the game plugins.
Cleanup|libcommon|All Games: Removed redundant common declarations

The functions is r_defs.h are really defined in the libcommon
p_mapsetup.cpp, so they should have a common header, too.
Cleanup|Refactor|All Games: Common config variables in one struct

The cfg.common struct now contains all the config variables that are
common to all games (most handled by libcommon).

Also removed some unused members from libhexen's config, which
revealed a use of cfg.snd3D which is no longer in use.
libcommon|Refactor: Unified the mobj thrust multiplier calculation

Since the thrust multiplier is directly related to friction, it should
be placed next to the friction calculations.
libcommon|Refactor: Added a common console registration function

Common variables should be registered here.