archive_plugin: wrap method calls

Make archive_file a "real" struct, extended by all plugins.  Add the
plugin pointer to it.  Wrap all method calls in functions.

src/archive/bz2_archive_plugin.c

@@ -40,6 +40,8 @@
 #define BZ_BUFSIZE   5000
 
 struct bz2_archive_file {
+	struct archive_file base;
+
 	char *name;
 	bool reset;
 	struct input_stream istream;
@@ -102,6 +104,7 @@ bz2_open(const char *pathname)
 	int len;
 
 	context = g_malloc(sizeof(*context));
+	archive_file_init(&context->base, &bz2_archive_plugin);
 
 	//open archive
 	if (!input_stream_open(&context->istream, pathname, NULL)) {
@@ -118,7 +121,7 @@ bz2_open(const char *pathname)
 		context->name[len - 4] = 0; //remove .bz2 suffix
 	}
 
-	return (struct archive_file *) context;
+	return &context->base;
 }
 
 static void

src/archive/iso9660_archive_plugin.c

@@ -35,6 +35,8 @@
 #define CEILING(x, y) ((x+(y-1))/y)
 
 struct iso9660_archive_file {
+	struct archive_file base;
+
 	iso9660_t *iso;
 	iso9660_stat_t *statbuf;
 	size_t cur_ofs;
@@ -93,6 +95,8 @@ iso9660_archive_open(const char *pathname)
 	struct iso9660_archive_file *context =
 		g_new(struct iso9660_archive_file, 1);
 
+	archive_file_init(&context->base, &iso9660_archive_plugin);
+
 	context->list = NULL;
 
 	/* open archive */
@@ -104,7 +108,7 @@ iso9660_archive_open(const char *pathname)
 
 	listdir_recur("/", context);
 
-        return (struct archive_file *)context;
+	return &context->base;
 }
 
 static void

src/archive/zzip_archive_plugin.c

@@ -32,6 +32,8 @@
 #include <string.h>
 
 struct zzip_archive {
+	struct archive_file base;
+
 	ZZIP_DIR *dir;
 	ZZIP_FILE *file;
 	size_t	length;
@@ -55,6 +57,8 @@ zzip_archive_open(const char *pathname)
 	struct zzip_archive *context = g_malloc(sizeof(*context));
 	ZZIP_DIRENT dirent;
 
+	archive_file_init(&context->base, &zzip_archive_plugin);
+
 	// open archive
 	context->list = NULL;
 	context->dir = zzip_dir_open(pathname, NULL);
@@ -71,7 +75,7 @@ zzip_archive_open(const char *pathname)
 		}
 	}
 
-	return (struct archive_file *)context;
+	return &context->base;
 }
 
 static void