findSpawnPos: Add setting for max height above water level

Increase default from 6 to 16 to help with mgv7 and mgfractal
Large-scale or alternative mapgens can result in a lowland spawn point not
being found, causing a spawn at (0, 0, 0) possibly buried underground
The max height is now settable to allow correct player spawn
in any mapgen or when using custom noise parameters

minetest.conf.example

@@ -829,6 +829,13 @@
 #    type: string
 # static_spawnpoint = 
 
+#    Maximum distance above water level for player spawn.
+#    Larger values result in spawn points closer to (x = 0, z = 0).
+#    Smaller values may result in a suitable spawn point not being found,
+#    resulting in a spawn at (0, 0, 0) possibly buried underground.
+#    type: int
+# vertical_spawn_range = 16
+
 #    If enabled, new players cannot join with an empty password.
 #    type: bool
 # disallow_empty_password = false