Refactor nanosleep in terms of clock_nanosleep

The generic version is straightforward.  For Hurd, its nanosleep
implementation is moved to clock_nanosleep with adjustments from
generic unix implementation.

The generic clock_nanosleep unix version is also removed since
it calls nanosleep.

Checked on x86_64-linux-gnu and powerpc64le-linux-gnu.

Reviewed-by: Florian Weimer <fweimer@redhat.com>