Set sky API: Add bool for clouds in front of custom skybox

Default true.
Add 'm_clouds_enabled' bool to sky.h, set from new bool in 'set sky' API.
Make 'getCloudsVisible()' depend on 'm_clouds_enabled' instead of
'm_visible' (whether normal sky is visible).

src/network/networkprotocol.h

@@ -584,6 +584,7 @@ enum ToClientCommand
 		foreach count:
 			u8 len
 			u8[len] param
+		u8 clouds (boolean)
 	*/
 
 	TOCLIENT_OVERRIDE_DAY_NIGHT_RATIO = 0x50,