Kernel: Add a method to determine the desired permissions of a Device

This method will be used later in DevFS, to set the appropriate
permissions for each device node.

Kernel/TTY/MasterPTY.h

@@ -48,6 +48,9 @@ public:
 
     virtual String absolute_path(const FileDescription&) const override;
 
+    // ^Device
+    virtual mode_t required_mode() const override { return 0640; }
+
 private:
     // ^CharacterDevice
     virtual KResultOr<size_t> read(FileDescription&, size_t, UserOrKernelBuffer&, size_t) override;