Added permission checks for chmod, link, rename and chown

Those clearly need to ask for permissions.

src/sourcefs.h

@@ -47,6 +47,8 @@ int source_chown(const char *filename, uid_t owner, gid_t group);
 
 int source_truncate(const char *filename, off_t length);
 
+int source_access(const char *filename, int mode);
+
 /* `open` and `create` are designed to correspond to fuse operations, not the
  * libc's `open(2)`. Both of them actually call `openat`. */