Add ability to detect CGame support for tab completion at runtime

[dGr8LookinSparky]

If on a server that doesn't support tab completion for /callvote, tab completion is attempted, the client goes back to the main menu with the following error:

vmMain: unknown command 11

[wtfbbqhax]

Ya, there was supposed to be a new cvar or something similar to toggle the functionality. I might consider disabling it all together until that's addressed since this is really annoying.

[wtfbbqhax]

I've disabled the feature for the time being.

Repurposing this bug to address adding the missing functionality so that we can re-enable the feature.