Changes and fixes to how gamemodes and antags are chosen.

Antags that are unrelated to the jobs and malfunctioning AI (Malf, Blob, Nuke, Wizard) are now chosen prior to job selection. This means you will have an equal chance to be a malf. AI if you have that turned on (Fix to issue #1127).
Fix to rare cases in which you would have a game start with less then three head revs or cultists (game will go back to lobby in this case).
People jobbanned from a certain antag position are now never considered for it.
can_start() is no longer directly called if the mode is secret since get_runnable_modes() does that for you anyway.
Added a message for admins if forcing the secret game mode was not successful.

What could still be done:
Move all antag selections prior to job selection for truly equal opportunity murderboner.
Proper enforcement of player antag preferences (currently any gamemode beside malf will select players without the special role candidacy on as a last resort).