Change the group and the permissions on /dev/acpi, to make "acpiconf"
work when called by members of the 'operator' group. They are already
allowed to eg power off the system (via suid shutdown(8)), so they
might as well be permitted to suspend it.

Tested by: xmj@
Reviewed by: delphij@
MFC after: 2 weeks
Relnotes: yes
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16062