Fix spectating after disconnection of spectated player

Before if you watched player from the first spectating position
(SM_FOLLOW1ST), and that player disconnected, then you switched
automatically to another player, but also to second spectating position
(SM_FOLLOW3RD), instead of stay in SM_FOLLOW1ST - this commit fixes that.
Allow persistent spectating concrete player in multiplayer

For spectatepersistent = 1 that is always enabled (that means, that
spectated player isn't changed after his death), for 0  - only in
non-arena modes, except of demos.
"if(!available.length()) return f;" is introduced, in order to, in arena
modes, doesn't happen switching to SM_FLY, when last alive players dies
in the same moment and there is no winner of the round.