packages: Move powerd to its own package

Nearly everyone wants powerd to manage CPU frequency scaling on real
hardware, even if -utilities isn't installed. Conversely, very small
jails might want -utilities but don't need powerd.

Move it to its own package and add it to the minimal set.

MFC after: 1 day
Reviewed by: manu, imp
Differential Revision: https://reviews.freebsd.org/D52645