Fix canonicalize loop-detection corner case.

Fix canonicalize loop-detection corner case.

Do not attempt to stat the symlink values stored via seen_triple.
Without this, coreutils' tests/misc/readlink-fp-loop test would fail
on linux-2.6.18, (but not 2.6.22).
* lib/canonicalize.c (seen_triple): Use triple_compare_ino_str, not
triple_compare.  The former compares dev,ino,filename, while the latter
would actually stat dirname(filename) when dev and ino were equal.
* lib/hash-triple.c: Install <string.h>.
(STREQ): Define.
(triple_compare_ino_str): New function.
* lib/hash-triple.h (triple_compare_ino_str): Declare it.