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.
Fix spectating after disconnection of spectated player

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