tar: fix and sanitize handling of long filenames/linknames

(GNU extensions 'K' and 'L').
We correctly handle them when untarring now, but unfortunately
we still don't use them when tarring! That stupid 100 char limit
is still there!
The biggest problem is that we don't support 'pax' tar format.
Linux kernel tarballs are in this format... shame

libbb/xfuncs.c

@@ -74,6 +74,7 @@ char * xstrndup(const char *s, int n)
 	if (ENABLE_DEBUG && s == NULL)
 		bb_error_msg_and_die("xstrndup bug");
 
+	/* TODO: think about xstrndup("abc", 10000)!!! */
 	t = xmalloc(++n);
 
 	return safe_strncpy(t,s,n);