Add 'spawn' mod to spawn new players in suitable starting biomes (#2091)

Disabled in mgv6 and singlenode mapgens, by setting, or if
'static_spawnpoint' is set.

Cleanup format of minetest.conf.example.

settingtypes.txt

@@ -46,3 +46,7 @@ share_bones_time (Bone share time) int 1200 0
 
 #    Replaces old stairs with new ones. Only required for older worlds.
 enable_stairs_replace_abm (Replace old stairs) bool false
+
+#    If enabled, use the engine's spawn search which does not check for a
+#    suitable starting biome.
+engine_spawn (Use engine spawn search) bool false