Make the clock routines bmk_platform_cpu_clock_foo

bmk can now directly call into the relevant code instead of a bounce.

clock routines don't belong in sched.h, move to platform.h

Retire sched_nanosleep(), blockprepare_timeout() is sufficient

Rename bmk_clock_*() to bmk_platform_clock_*()

They are expected to be implemented by the underlying platform.

Rename librumprun_rumpuser to libbmk_rumpuser

Layeristically speaking, the common rumpuser bits run on top of bmk,
so the renamed name now also reflects reality (and matches the
header namespace).