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>

crtools.c

@@ -182,6 +182,18 @@ struct cr_fd_desc_tmpl fdset_template[CR_FD_MAX] = {
 		.magic	= FS_MAGIC,
 		.show	= show_fs,
 	},
+
+	[CR_FD_REMAP_FPATH] = {
+		.fmt	= FMT_FNAME_REMAP_FPATH,
+		.magic	= REMAP_FPATH_MAGIC,
+		.show	= show_remap_files,
+	},
+
+	[CR_FD_GHOST_FILE] = {
+		.fmt	= FMT_FNAME_GHOST_FILE,
+		.magic	= GHOST_FILE_MAGIC,
+		.show	= show_ghost_file,
+	},
 };
 
 static struct cr_fdset *alloc_cr_fdset(int nr)