Add overtime when round ends in a tie

[GooberRF]

This PR adds sudden death overtime as an optional feature for server operators. If enabled, when the time limit is reached, if the score is a tie, some additional time (configurable) will be added and the game will end either when that additional time has elapsed, or the tie is broken (whichever comes first).

Configuration of the feature in dedicated_server.txt is as follows (default values shown):

    $DF Overtime Enabled: false
        +Duration: 5

"Ties" for the purpose of overtime are defined as:

• DM: At least 2 players are tied for the highest score.
• TDM: Red and blue have the same score.
• CTF: Red and blue have the same number of captures OR the delta is 1, and a flag is currently stolen which, if captured, would tie the game.

(in the case of CTF, if the game is tied because a flag is stolen and would tie the game if captured, the tie is broken if that flag is returned)

A few other minor points:

• I had to reimplement most of multi_check_for_round_end_hook to support overtime either way, but I also took the opportunity to restructure the code using modern C++ methods as opposed to the stock game implementation.
• Unlike the stock code, my new code no longer continually attempts to initiate a level change every frame after the round ends. In theory this should be a performance improvement but I've not attempted to measure it or anything.