Allow areas to be optionally open to all other users

In other words, you can retain ownership of an area but disable and
re-enable the protection aspect at will, via the area_open chat command.

By default, nothing is any different following this commit - all
existing areas are protected, as are new ones. But you can do (for
example) "area_open 1" and if you're the owner of that area, it's
now possible for other users to interact there. "area_open 1" again to
toggle the protection back on.

Where there are sub-areas, the main owner and ALL sub-area owners at a
particulare location must have set the areas to open to disable
protection.

hud.lua

@@ -15,7 +15,11 @@ minetest.register_globalstep(function(dtime)
 			else
 				first = false
 			end
-			areaString = areaString..id.." ("..area.owner..")"
+			local ownertxt = area.owner
+			if area.open then
+				ownertxt = ownertxt.."/open"
+			end
+			areaString = areaString..id.." ("..ownertxt..")"
 		end
 		if not areas.hud[name] then
 			areas.hud[name] = {}