POSIX-compliant version of fchmodat with flag argument

Hi,

The below patches implement a 4 argument version of fchmodat (fchmodat4)
that has a flag argument, as specified by POSIX.  This is needed to
implement a proper glibc wrapper.

fchmodat4 supports the same two flags as fchownat: AT_SYMLINK_NOFOLLOW
and AT_EMPTY_PATH.

Besides the POSIX-compliance, this patch will make it possible to
ensure, in a race-free way, that you do not follow symlinks when
chmodding. Previously, you could open a file with O_NOFOLLOW and fchmod
it, but this only worked if you had read or write permissions on the
file.  Now you can open the file with O_PATH|O_NOFOLLOW and do fchmodat
with AT_EMPTY_PATH.

Could this patch be applied?  Any feedback is welcome.

Cheers, 
Andrew
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