Add API to control respawn logic and behavior

[sfan5]

resolves #2630, resolves #2117

How to test

1. Carefully inspect code
2. Check that bed respawning works
3. Try the static_spawnpoint, engine_spawn and enable_bed_respawn settings
4. Also check that it works as expected on mgv6 (spawn algo is disabled there)

[appgurueu]

code looks fine

tested on v6:

• spawns me at the world origin by default;
• static_spawnpoint works;
• engine_spawn works and finds an alternative spawnpoint nearby if the world origin is obstructed
• bed respawning works

tested on v7:

• static_spawnpoint works;
• engine_spawn works;
• bed respawning works