Create an ApplyDevices call.

For our work on adding dynamic device support to Docker we needed to be
able to call this to update the list of allowed devices.  This works for
both systemd and fs based cgroups implementations.

Co-Authored-By: Chris Alfonso <calfonso@redhat.com> (github: calfonso)
Docker-DCO-1.1-Signed-off-by: Ian Main <imain@redhat.com> (github: imain)
Merge pull request #209 from imain/publicjoin

Make joinDevices public.