directory: added "mtime" property

Remember the modification time of each directory.  This is important
for archives (which are virtual directories right now), but may also
be useful for an automatic update mechanism.

src/update.c

@@ -360,10 +360,12 @@ update_archive_tree(struct directory *directory, char *name)
  *
  * @param parent the parent directory the archive file resides in
  * @param name the UTF-8 encoded base name of the archive file
+ * @param st stat() information on the archive file
  * @param plugin the archive plugin which fits this archive type
  */
 static void
 update_archive_file(struct directory *parent, const char *name,
+		    const struct stat *st,
 		    const struct archive_plugin *plugin)
 {
 	char *path_fs;
@@ -393,6 +395,8 @@ update_archive_file(struct directory *parent, const char *name,
 		directory->device = DEVICE_INARCHIVE;
 	}
 
+	directory->mtime = st->st_mtime;
+
 	plugin->scan_reset(file);
 
 	while ((filepath = plugin->scan_next(file)) != NULL) {
@@ -438,7 +442,7 @@ update_regular_file(struct directory *directory,
 		}
 #ifdef ENABLE_ARCHIVE
 	} else if ((archive = archive_plugin_from_suffix(suffix))) {
-		update_archive_file(directory, name, archive);
+		update_archive_file(directory, name, st, archive);
 #endif
 	}
 }
@@ -593,6 +597,8 @@ updateDirectory(struct directory *directory, const struct stat *st)
 
 	closedir(dir);
 
+	directory->mtime = st->st_mtime;
+
 	return true;
 }