cvar: Use the generic linked list for cvars. Made virtual cvars actual cvar_t's

cvar: Use the generic linked list for cvars. Made virtual cvars actual cvar_t's

* cvar_hash_t is gone
* Virtual cvars now use a linked list around their parent and have a
  pointer to their parent.
* Cvars will now store their own hashindex to make deletions from the
  hashtable easier. Implemented Cvar_Delete where this is leveraged.
  Cvar_FindVarLink is eliminated also. Not needed anymore.
* Implemented Cvar_DeleteVirtual and Cvar_DeleteVirtual_All
  to... delete virtual cvars.
* Eliminated redundancy of Cvar_SetQuick_Internal by merging it with
  Cvar_SetQuick.
* Misc cleanups