docker daemon: initialize the daemon pidfile early

fixes https://github.com/dotcloud/docker/issues/6973

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Update docs on --sig-proxy to indicate that SIGKILL and SIGSTOP cannot be proxied

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)
Merge pull request #7104 from mheon/sigproxy_docs

Update docs on --sig-proxy
Merge pull request #6976 from vbatts/vbatts-early_daemon_pidfile

docker daemon: initialize the daemon pidfile early