QGL renundancy cleanup

Despite QGL is only handling GL extensions, there was a little code renundancy.
QGL_EXT_Reset sets all extension pointers to NULL.
Elaborate on 64 bit windows builds
Make ARCH overrideable on Windows
Rename CDA_ENABLED to CDA_DISABLED. Some minor wording fixes while here.
Merge pull request #28 from alericoveri/qgl_cleanup

QGL renundancy cleanup
Move entitycmpfnc() to cl_view.c an reimplement it

This function is only used in cl_view.c, so no need for external
declaration. Reimplement it in a sane and on all platform 64 bit
clean way. This allows us to finally remove the horible INT macro.
Do not set the FPU to single precision on Windows

We had dual precision since ever on 64 bit unixoid systems and it "just
worked". So there should be no reason to still force Windows down to
single precision.  The performance impact should be negligible.
Set an rpath entry to WITH_SYSTEMDIR/lib

This automagically sets an rpath to WITH_SYSTEMDIR/lib, when
WITH_SYSTEMWIDE is specified. If no WITH_SYSTEMDIR is given,
the default path /usr/share/games/quake2/lib is used. As for
the normal rpath, for now only FreeBSD and Linux are supported.
This was requested by caedes.
Wrap too long line
Add an signal handler for SIGINT and SIGTERM

This allows the user to shut the client down by pressing ctrl-c in it's
terminal or by sendig SIGTERM.
Update CHANGELOG for 5.21
Bump version number to 5.21
Fix a (harmless) off-by-one in a SDL_VideoDriverName() call
Refactor the SDL input backend once again.

- Remove the keyq. This linked list was used to work around limitations
  of the good, old DirectInput API. It's not necessary with SDL.
- Inline some functions to others where apropriate and remove unused
  or unnecessary functions.
- Reorder functions into groups.
- Update comments.

This commits still misses necessary changes to the frontend. They'll be
done in subsequential commits.
Do not allow input devices to write to command buffer

Input devices should send key events and nothing more. The ability to
add commands into the input buffer was used by the joystick code
(removed long time ago) and as a dirty hack to work around limitations
of DirectInput.
Remove unnecessary IN_Close()

Having a special function to close the input devices is unnecessary. The
backend should close them at shutdown, when IN_BackendShutdown() is
called.
Merge input backend init and keyboard init

Now that the refresher is part of the client, there's no need to init
the backend before the refresher and the keyboard after it.
Fix key-repeats in the console (for instance the backspace key)
Whitespace
Minor input grabbing cleanup.