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)
Add Content-Type header in PushImageLayerRegistry

Docker-DCO-1.1-Signed-off-by: Gabor Nagy <mail@aigeruth.hu> (github: Aigeruth)
Added :neckbeard: RMS the founder of :water_buffalo: GNU and :free: as in :free:dom software

Docker-DCO-1.1-Signed-off-by: Aleksandrs Fadins <aleks@s-ko.net> (github: muchweb)
Removed :neckbeard: RMS test case

Docker-DCO-1.1-Signed-off-by: Aleksandrs Fadins <aleks@s-ko.net> (github: muchweb)
archive/tar: update to fix writing of PAX headers

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
contrib/init/systemd: add philips as MAINTAINER

As requested after #7021 add me as a maintainer alongside the sword
toting @lsm5.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Updated Red Hat Enterprise Linux installation instructions for version 7

Docker-DCO-1.1-Signed-off-by: Brian Exelbierd <bex@pobox.com> (github: bcexelbi)
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)
Improve data volume container example.
Docker-DCO-1.1-Signed-off-by: Jason Livesay <ithkuil@gmail.com> (github: runvnc)
Remove blank "use" documentation

Introduced in dotcloud/docker#3070

Docker-DCO-1.1-Signed-off-by: Ben Firshman <ben@firshman.co.uk> (github: bfirsh)
Give context for the error that occures when you try to bind mount a volume with a relative path

Docker-DCO-1.1-Signed-off-by: Timothy <timothyhobbs@seznam.cz> (github: timthelion)
Update dockerlinks.md

Added missing command for logging into web container.
Move TestCopyVolumeUidGid to integration-cli

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Move TestCopyVolumeContent to integration-cli

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Merge pull request #7114 from bfirsh/remove-blank-use-doc

Remove blank "use" documentation
Adjust Travis to also compile, to test that compilation still succeeds

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #7117 from hipertracker/patch-1

Update dockerlinks.md
Fix minor grammar issues in dockervolumes.md
Merge pull request #7131 from msabramo/patch-1

Fix minor grammar issues in dockervolumes.md
Fix a typo in the tutorial