chmod --cheap option

[Hrubák , András]

Dear Coreutils Dev Team,

I made a patch for chmod.
It introduces an option `--cheap' to avoid real chmod (fchmodat) call
if the new mode seems equivalent to the old one.
It not only saves IO time, but also leaves ctime unchanged. While
ctime often plays role in updating/backuping/cleaning mechanisms, I
needed such a feature.
Please see it here: https://github.com/coreutils/coreutils/pull/6