If writing the CPPC_REQUEST MSR fails, make sure we do not set the
intended new value in the 'cppc.request' field of the softc as it was
not installed. Both set_cppc_request_cb() and enable_cppc_cb() were
changed to this effect.

In case enable_cppc_cb() could not read CPPC_REQUEST, mark that through
a new softc flag, HWPFL_CPPC_REQUEST_NOT_READ, so that we do not keep
and use a wrong value when the content of CPPC_REQUEST is read/written
through sysctl(9) knobs, but instead retry reading the MSR (this is the
purpose of the new get_cppc_request() sub-function).

When setting CPPC_REQUEST has failed, distinguish the case where it
could not be read at all from the case where it could not be written, by
respectively returning EIO and EOPNOTSUPP in these cases. The previous
return value of EFAULT was confusing as sysctl(3) documents it as
happening if the passed arguments are invalid.

While here, add some herald comment before sysctl_cppc_dump_handler()
clarifying that the intent of this function is to always query the
hardware directly, bypassing any cached value in the softc.