luci-base: dispatcher.lua: support "absent" fs dependency for menu nodes

The "absent" dependency type requires the given path to not exist on the
local system for the condition to be satisified. This is useful to disable
menu nodes depending on the presence of specific files.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 2e4b5fb8ff2fb3bfd1400bf7cbf721057f02fab9)

modules/luci-base/luasrc/dispatcher.lua

@@ -39,6 +39,10 @@ local function check_fs_depends(spec)
 			if fs.stat(path, "type") ~= "reg" then
 				return false
 			end
+		elseif kind == "absent" then
+			if fs.stat(path, "type") then
+				return false
+			end
 		end
 	end