Added 7 QC files from the GPL'd Quake 1 source code, plus 'progs.src'.
This is the bare minimals, just enough functionality to be able to enter
a map and walk around.  I will trim even more of the fat shortly....
Game : added dpdefs/progsdefs.qc to 'progs.src', and hence removed the
equivalent definitions from plain the 'defs.qc' file.
Makefile : build the game program (oga/progs.dat) in 'all' target.
Game : fixed numerous compiler warnings.
Data : added basic 'quake.rc' file (executed by engine during startup).
Data : added fairly basic 'default.cfg' file -- with some default values for
important cvars, plus some default key / mouse bindings.
README.md : noted quake.rc
Data : added three MDL models from OpenQuartz : progs/player.mdl,
progs/v_axe.mdl and progs/v_shot.mdl.  These are under the GNU GPL
license.  They are only here as place-holders, to allow the engine
to run, and will be removed when they are no longer necessary.
Data : added 'textures/stone3.tga' and a normal map.  This texture was
sourced from OpenGameArt by author 'Spiney' and under the CC0 license.
For now it is just a place-holder -- something to let the engine run.