Draft: Refactor automatic mode lockon (resolves #751)

This makes automatic mode lockon rely on a player flag instead of a cvar and fixes 3 issues:

1. Toggle mode weird behavior when quitting the game (#751 (closed))
2. Getting stuck in lockon mode if you switched control schemes while locked on
3. Lockon in general not properly clearing when switching map, returning to title, or quitting the game

Note that this also includes !1704 (merged) for the sake of testing

Test exe: srb2test-lockon.exe