cpio: fix a bug where we do not extract zero-sized hardlinks

(spotted at http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=466771).
Add testsuite entry for that, and another one for another bug:
we do not list hardlinks in cpio -t (not fixed).

function                                             old     new   delta
get_header_cpio                                      884     909     +25
static.saved_hardlinks_created                         -       4      +4
static.pending_hardlinks                               4       -      -4
static.inode                                           4       -      -4
cpio_main                                           1122    1060     -62

archival/libunarchive/seek_by_jump.c

@@ -9,11 +9,9 @@
 void seek_by_jump(const archive_handle_t *archive_handle, unsigned amount)
 {
 	if (lseek(archive_handle->src_fd, (off_t) amount, SEEK_CUR) == (off_t) -1) {
-#if ENABLE_FEATURE_UNARCHIVE_TAPE
-		if (errno == ESPIPE) {
+		if (errno == ESPIPE)
 			seek_by_read(archive_handle, amount);
-		} else
-#endif
+		else
 			bb_perror_msg_and_die("seek failure");
 	}
 }