[cvar] Make cvars properly typed

This is an extremely extensive patch as it hits every cvar, and every
usage of the cvars. Cvars no longer store the value they control,
instead, they use a cexpr value object to reference the value and
specify the value's type (currently, a null type is used for strings).
Non-string cvars are passed through cexpr, allowing expressions in the
cvars' settings. Also, cvars have returned to an enhanced version of the
original (id quake) registration scheme.

As a minor benefit, relevant code having direct access to the
cvar-controlled variables is probably a slight optimization as it
removed a pointer dereference, and the variables can be located for data
locality.

The static cvar descriptors are made private as an additional safety
layer, though there's nothing stopping external modification via
Cvar_FindVar (which is needed for adding listeners).

While not used yet (partly due to working out the design), cvars can
have a validation function.

Registering a cvar allows a primary listener (and its data) to be
specified: it will always be called first when the cvar is modified. The
combination of proper listeners and direct access to the controlled
variable greatly simplifies the more complex cvar interactions as much
less null checking is required, and there's no need for one cvar's
callback to call another's.

nq-x11 is known to work at least well enough for the demos. More testing
will come.

qw/include/sv_demo.h

@@ -22,9 +22,9 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 #define __sv_demo_h
 
 extern struct recorder_s demo;
-extern struct cvar_s *sv_demofps;
-extern struct cvar_s *sv_demoPings;
-extern struct cvar_s *sv_demoMaxSize;
+extern float sv_demofps;
+extern float sv_demoPings;
+extern char *sv_demoMaxSize;
 
 void SV_Stop (int reason);
 void Demo_Init (void);