fchmodat: port to old Linux kernel + newer headers
Problem reported by Lance Fredrickson in:
https://lists.gnu.org/r/bug-gnulib/2022-06/msg00038.html
* lib/fchmodat.c (fchmodat):
* lib/lchmod.c (lchmod): Do not rely on AT_EMPTY_PATH as to
whether syscalls work on ""; instead, if a call fails with
ENOENT assume that those syscalls do not work.
Do not use fstatat to determine whether a file is a symlink,
as this has problems with EOVERFLOW. Use readlinkat instead,
and if it fails with EINVAL then the file is not a symlink.
Remove #if tests on __linux__ || __ANDROID__ || __CYGWIN__
as this has been a maintenance hassle and it’s unlikely
these days that a new platform would #define O_PATH without also
either supporting /proc or keeping it absent.
* modules/fchmodat (Depends-on): Remove fstatat.
There should be no need for either fchmodat or lchmod to depend on
readlinkat, since they use readlinkat only in contexts where it
should work without Gnulib intervention.