docker daemon: initialize the daemon pidfile early

fixes https://github.com/dotcloud/docker/issues/6973

This way there are no races on iptables, etc. before there is a discovery of existing daemon pidfile

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
understood, but we'll need to keep compatibility with someone calling `docker -d -p ""`. Perhaps, just just don't init that job, if pidfile is ""