files: Support dumping/restoring of completely unlinked files

Completely unlinked file is the one with n_link count being zero.
Such files only allow to read their contents and carry with us.

In order to dump this thing I introduce the "path remap" technology.
For reg file a remapping entry is dumped which describes, that at
restore stage before opening a regfile->path this path should be
linked to some other name and then (after open) unlinked.

For completely unlinked files the remap path would be a path to
a "ghost" file, i.e. a file which is created only at the time of
restore and which is removed completely at the end of it.

Partially unlinked files (i.e. those having n_link != 0, but a
path by which we see them in someone's fd is not accessible) should
be handled in another way.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/files.h

@@ -73,4 +73,6 @@ struct file_desc;
 extern int collect_pipes(void);
 extern void mark_pipe_master(void);
 
+void clear_ghost_files(void);
+
 #endif /* FILES_H_ */