Abstract read and seek in unarchiving code, convert bunzip to file descriptors, support tar -j

archival/libunarchive/get_header_tar_gz.c

@@ -29,7 +29,10 @@ extern char get_header_tar_gz(archive_handle_t *archive_handle)
 	int pid;
 	unsigned char magic[2];
 	
-	xread_all(archive_handle->src_fd, &magic, 2);
+	/* Cant lseek over pipe's */
+	archive_handle->seek = seek_by_char;
+
+	archive_xread_all(archive_handle, &magic, 2);
 	if ((magic[0] != 0x1f) || (magic[1] != 0x8b)) {
 		error_msg_and_die("Invalid gzip magic");
 	}