Support hairpin NAT without going through docker server

Hairpin NAT is currently done by passing through the docker server.  If
two containers on the same box try to access each other through exposed
ports and using the host IP the current iptables rules will not match the
DNAT and thus the traffic goes to 'docker -d'

This change drops the restriction that DNAT traffic must not originate
from docker0.  It should be safe to drop this restriction because the
DOCKER chain is already gated by jumps that check for the destination
address to be a local address.

Docker-DCO-1.1-Signed-off-by: Darren Shepherd <darren.s.shepherd@gmail.com> (github: ibuildthecloud)
Return error when existing bridge does not match ip
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Follow symlinks inside container root for build's ADD
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Strip comments before parsing line continuations

Fixes #3898
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Fix commit and import when no repository is specified
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Adjust TestOnlyLoopbackExistsWhenUsingDisableNetworkOption to ignore "DOWN" interfaces

This fixes the following, which I've been seeing on all my machines for as long as I can remember:

--- FAIL: TestOnlyLoopbackExistsWhenUsingDisableNetworkOption (0.36 seconds)
	container_test.go:1597: Wrong interface count in test container: expected [*: lo], got [1: lo 2: sit0]

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Add --opt arguments for drivers

In order to handle special configuration for different drivers we
make the Config field a map to string array. This lets
us use it for lxc, by using the "lxc" key for those, and we can
later extend it easily for other backend-specific options.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
cgroups: Splity out Apply/Cleanup to separate file/interface

This leaves only the generic cgroup helper functions in cgroups.go and
will allow easy implementations of other cgroup managers.

This also wires up the call to Cleanup the cgroup which was missing
before.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
cgroups: Join groups by writing to cgroups.procs, not tasks

cgroups.procs moves all the threads of the process, and "tasks" just
the one thread. I believe there is a risk that we move the main thread,
but then we accidentally fork off one of the other threads if the go
scheduler randomly switched to another thread. So, it seems safer (and
more correct) to use cgroups.procs.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Vendor github.com/godbus/dbus and github.com/coreos/go-systemd

We need this to do systemd API calls.

We also add the static_build tag to make godbus not use
os/user which is problematic for static builds.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
pkg/systemd: Drop our copy-pasted version of go-systemd/activation

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Add systemd.SdBooted()

This is a conversion of sd_booted() from libsystemd to go and checks
if the system was booted with systemd.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
cgroups: Add systemd implementation of cgroups

This implements cgroup.Apply() using the systemd apis.
We create a transient unit called "docker-$id.scope" that contains
the container processes. We also have a way to set unit specific
properties, currently only defining the Slice to put the
scope in.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4871 from crosbymichael/bip-exists

Return error when existing bridge does not match ip
Merge pull request #4873 from crosbymichael/add-internal-symlinks

Follow symlinks inside container root for build's ADD
Merge pull request #4450 from alexlarsson/cgroups-systemd

Add systemd implementation of cgroups
Merge pull request #4442 from ibuildthecloud/hairpin-nat

Support hairpin NAT without going through docker server
Merge pull request #4889 from crosbymichael/fix-commit-validation

Fix commit and import when no repository is specified
Merge pull request #4880 from tianon/ignore-down-interfaces

Adjust TestOnlyLoopbackExistsWhenUsingDisableNetworkOption to ignore "DOWN" interfaces
Merge pull request #4759 from crosbymichael/comments-in-buildfile

Strip comments before parsing line continuations