Disallow digging a bed if in use

Use a new table beds.bed_position to check if a bed is in use.

game_api.txt

@@ -42,6 +42,7 @@ Beds API
 		def            -- See [#Bed definition]
 	)
 
+ * `beds.can_dig(bed_pos)` Returns a boolean whether the bed at `bed_pos` may be dug
  * `beds.read_spawns() `   Returns a table containing players respawn positions
  * `beds.kick_players()`  Forces all players to leave bed
  * `beds.skip_night()`   Sets world time to morning and saves respawn position of all players currently sleeping