update seamless uncompression code

This change makes "tar tf hello_world.txz" work without
adding special-casing for ".txz" extension. It also removes
ever-growing magic checking code in rpm2cpio and get_header_tar -
we reuse one which lives in setup_unzip_on_fd.

function                                             old     new   delta
unpack_gz_stream                                       7     566    +559
check_signature16                                      -      70     +70
setup_unzip_on_fd                                     99     142     +43
handle_SIGCHLD                                         -      41     +41
unpack_bz2_stream                                    342     376     +34
unzip_main                                          2352    2385     +33
bbunpack                                             503     533     +30
open_transformer                                      74     102     +28
unpack_Z_stream                                     1278    1304     +26
unpack_gunzip                                        101     123     +22
init_transformer_aux_data                              -      18     +18
unpack_xz_stream                                    2388    2402     +14
open_zipped                                          131     141     +10
rpm_main                                            1358    1363      +5
get_header_tar_lzma                                   52      57      +5
get_header_tar_bz2                                    52      57      +5
unpack_lzma_stream                                  2698    2702      +4
hash_find                                            234     233      -1
get_header_tar                                      1759    1733     -26
get_header_tar_gz                                     92      57     -35
unpack_uncompress                                     51      12     -39
rpm2cpio_main                                        201     147     -54
unpack_unxz                                           67      12     -55
unpack_bz2_stream_prime                               55       -     -55
get_header_tar_Z                                      86       -     -86
unpack_gz_stream_with_info                           539       -    -539
------------------------------------------------------------------------------
(add/remove: 3/3 grow/shrink: 14/6 up/down: 947/-890)          Total: 57 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/archival/libarchive/decompress_bunzip2.c b/archival/libarchive/decompress_bunzip2.c
index c4640d4..dc252bb 100644
--- a/archival/libarchive/decompress_bunzip2.c
+++ b/archival/libarchive/decompress_bunzip2.c
@@ -721,7 +721,7 @@
 
 /* Decompress src_fd to dst_fd.  Stops at end of bzip data, not end of file. */
 IF_DESKTOP(long long) int FAST_FUNC
-unpack_bz2_stream(int src_fd, int dst_fd)
+unpack_bz2_stream(transformer_aux_data_t *aux, int src_fd, int dst_fd)
 {
 	IF_DESKTOP(long long total_written = 0;)
 	bunzip_data *bd;
@@ -729,6 +729,9 @@
 	int i;
 	unsigned len;
 
+	if (check_signature16(aux, src_fd, BZIP2_MAGIC))
+		return -1;
+
 	outbuf = xmalloc(IOBUF_SIZE);
 	len = 0;
 	while (1) { /* "Process one BZ... stream" loop */
@@ -794,17 +797,6 @@
 	return i ? i : IF_DESKTOP(total_written) + 0;
 }
 
-IF_DESKTOP(long long) int FAST_FUNC
-unpack_bz2_stream_prime(int src_fd, int dst_fd)
-{
-	uint16_t magic2;
-	xread(src_fd, &magic2, 2);
-	if (magic2 != BZIP2_MAGIC) {
-		bb_error_msg_and_die("invalid magic");
-	}
-	return unpack_bz2_stream(src_fd, dst_fd);
-}
-
 #ifdef TESTING
 
 static char *const bunzip_errors[] = {
@@ -819,7 +811,7 @@
 	int i;
 	char c;
 
-	int i = unpack_bz2_stream_prime(0, 1);
+	int i = unpack_bz2_stream(0, 1);
 	if (i < 0)
 		fprintf(stderr, "%s\n", bunzip_errors[-i]);
 	else if (read(STDIN_FILENO, &c, 1))