info_player_* triggering

[fnky]

Script types: All.

Allow handling upon activating the "checkpoints" in most maps, this would allow implementing survival mode in a much easier way on unsupported maps.

Potentially allow spawn blocking.

Note: info_player_start is treated by game code as though it inherits from the same base class as info_player_deathmatch and other info_player_* entities. Consider fixing this before adding the hook and exposing the base class for use here.