dpkg_deb: slight code shrink

ar: reuse existing ar unpacking code
get_header_tar: handle autodetection for tiny .tar.gz files too
unarchive.h: do not include CONFIGed out things

function                                             old     new   delta
get_header_tar                                      1521    1534     +13
dpkg_deb_main                                        400     380     -20
ar_main                                              260     196     -64
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/2 up/down: 13/-84)            Total: -71 bytes

1 files changed, 1 insertions, 9 deletions

diff --git a/archival/ar.c b/archival/ar.c
index d49329ce9..dbff6775e 100644
--- a/archival/ar.c
+++ b/archival/ar.c
@@ -45,7 +45,6 @@ int ar_main(int argc, char **argv)
 
 	archive_handle_t *archive_handle;
 	unsigned opt;
-	char magic[8];
 
 	archive_handle = init_handle();
 
@@ -82,14 +81,7 @@ int ar_main(int argc, char **argv)
 		llist_add_to(&(archive_handle->accept), argv[optind++]);
 	}
 
-	xread(archive_handle->src_fd, magic, 7);
-	if (strncmp(magic, "!<arch>", 7) != 0) {
-		bb_error_msg_and_die("invalid ar magic");
-	}
-	archive_handle->offset += 7;
-
-	while (get_header_ar(archive_handle) == EXIT_SUCCESS)
-		continue;
+	unpack_ar_archive(archive_handle);
 
 	return EXIT_SUCCESS;
 }