Add experimenta btrfs driver

This is an experimental btrfs driver. To use it you must have
/var/lib/docker mounted on a btrfs filesystem and explicitly
specify DOCKER_DRIVER=btrfs in the docker daemon environment.

It works by using subvolumes for the docker image/container layers.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
btrfs: Add comment to Put()

Document why we don't need to do anything in Put().

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
ExecStartPre commands updated

Docker-DCO-1.1-Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com> (github: lsm5)

systemd service no longer does '/bin/mount/ --make-rprivate /'.
Core issue fixed by Alex Larsson (commit 157d99a).

ip forwarding enabled.
remove ip fowarding from systemd (fixed: commit #3801)

Docker-DCO-1.1-Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com> (github: lsm5)
Merge pull request #3531 from lsm5/docker-systemd

ExecStartPre commands updated in systemd service file
contrib: systemd: make socket-activation Restart=always

Do as was done to f09a78cd219b24d4308034c8dd13410cfe5fbec7 in the
socket-activation example.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Merge pull request #3834 from philips/fix-socket-activation-systemd

contrib: systemd: make socket-activation Restart same as the main service file
Fix typo

Docker-DCO-1.1-Signed-off-by: Piergiuliano Bossi <pgbossi@gmail.com> (github: thinkingbox)
Merge pull request #3835 from thinkingbox/master

Fix typo
 Add troubleshooting for missing cgroups on mint 16 #3602

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
network: insert masq rule

This fixes IP masquerading on systems with reject rules at the end of
the POSTROUTING table, by inserting the rule at the beginning of the
table instead of adding it at the end.

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Merge pull request #3718 from alexlarsson/btrfs

Add btrfs support to docker
Make unclejack the official Vagrantfile maintainer

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3846 from tianon/unclejack-maintainer

Make unclejack the official Vagrantfile maintainer
Merge pull request #3836 from SvenDowideit/mint-cgroups-lite-docs

 Add troubleshooting for missing cgroups on mint 16
Merge pull request #3842 from jpoimboe/iptables-insert-masq

network: insert masq rule