Initial implementation of cvar aliases, with slowmo as a starting example

Initial implementation of cvar aliases, with slowmo as a starting example

This allows cvars to be renamed, and keep the old cvar by pointing it to
the new one. Pretty much every cvar finding function has been adapted to
return a the aliased cvar or the alias itself depending on context.

For example, if a value is needed, it'll dereference the alias. If the
cvar is needed to be listed somewhere or to check if the cvar merely
exists, it will not. Other than that, it's pretty straightforward.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12643 d7cf8633-e32d-0410-b094-e92efae38249