Add -N option to powerd(8) to ignore "nice" time.

With powerd_flags="-N", this makes powerd(8) exclude "nice" time when
computing the CPU utilization. This makes it possible to prevent
CPU-intensive "background" processes from spinning up the CPU.

Note that only *userland* CPU usage belonging to "nice" processes is
excluded; we do not track whether time spent in the kernel is on behalf
of nice or non-nice processes, so kernel-intensive nice processes can
still result in the CPU being sped up.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D23868