This commit adds the renice command and its man page,
it also introduces some fixes:
* Makes nice command more solid, it also makes it respect POSIX return values.
* Fixes estrtol, which produced a misleading error on out of range errors.
* Fixes chgrp.1 NAME section.
Signed-off-by: Christoph Lohmann <20h@r-36.net>
Lorenzo Cogotti