Adding file descriptor attribute(s) to gcc and glibc

[Florian Weimer]

* Szabolcs Nagy via Gcc:

> to be honest, i'd expect interesting fd bugs to be
> dynamic and not easy to statically analyze.
> the use-after-unchecked-open maybe useful. i would
> not expect the access direction to catch many bugs.

You might be right.  But I think the annotations could help to catch
use-after-close errors.

By the way, I think it would help us if we didn't have to special-case
AT_FDCWD using inline wrappers.

Thanks,
Florian