Improved default spawning

* Allowed mods to decide if spawnpoint is suitable or not
* Made so that player always spawns at first spawnpoint in singleplayer
maps, to prevent spawning too far in the map after a death
* Increased amount of tries to workout the "best" spawnpoint from 6 to
spawnpoint count
* Fixed all spawnpoints being suitable for a player without team (
TEAM_UNASSIGNED ). This caused players in sandbox sometimes spawn in
each other.
Merge pull request #535 from robotboy655/sandbox_spawn

Improved default spawning