New `kartgametypepreference` cvar.

* A "canon" adaptation of the community-created server option `lessbattlevotes`.
* If set to "None", voting behaves as before.
* If set to "Race" or "Battle". that gametype is considered the preference.
    * The voting screen is always operated from the perspective of the preferred gametype.
    * If you're in an un-preferred gametype, the third vote option will always allow you to continue the gametype.
    * If the preferred gametype is Race and you've just exited a Battle map, Encore may now appear on the second vote option.
* A number of bugs with voting have been corrected.
    * If `kartencore` is on, the third vote option will now correctly have Encore applied.
    * If a custom EXE or malformed packet sends an Encore flag alongside a Battle gametype ID, actively strip it.
        * Just to note, clients do not enter Battle Encore with or without this change - this just prevents a promise the rest of the game couldn't fulfill.