add open_nofollow() helper

Some callers of open() would like to use O_NOFOLLOW, but it is not
available on all platforms. Let's abstract this into a helper function
so we can provide system-specific implementations.

Some light web-searching reveals that we might be able to get something
similar on Windows using FILE_FLAG_OPEN_REPARSE_POINT. I didn't dig into
this further.

For other systems without O_NOFOLLOW or any equivalent, we have two
options for fallback:

  - we can just open anyway, following symlinks; this may have security
    implications (e.g., following untrusted in-tree symlinks)

  - we can determine whether the path is a symlink with lstat().

    This is slower (two syscalls instead of one), but that may be
    acceptable for infrequent uses like looking up .gitattributes files
    (especially because we can get away with a single syscall for the
    common case of ENOENT).

    It's also racy, but should be sufficient for our needs (we are
    worried about in-tree symlinks that we ourselves would have
    previously created). We could make it non-racy at the cost of making
    it even slower, by doing an fstat() on the opened descriptor and
    comparing the dev/ino fields to the original lstat().

This patch implements the lstat() option in its slightly-faster racy
form.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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