Add uid and gid to CharacterDevices.

The vast majority of them will be owned by 0:0 (the default.)
However, PTY pairs will now be owned by the uid:gid of the opening process.

Kernel/VirtualFileSystem.h

@@ -86,6 +86,8 @@ public:
 
     void sync();
 
+    CharacterDevice* get_device(unsigned major, unsigned minor);
+
 private:
     friend class FileDescriptor;