Bugfix: only use io.Copy in hijack if attaching both stdout and stderr

Add regression tests to ensure issue is fixed.

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)

Conflicts:
	integration-cli/docker_cli_run_test.go
Increase size of buffer for signals

The chan struct used to forward signals to containers was one element only,
which caused some signals to be dropped when many were being received.
Increasing the size of the chan buffer makes this much less likely to happen.

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)
Clear all capabilities even those that do not exist in capabilities.go

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Save "COMMENT" field in Dockerfile into image content.

This will allow a user to save COMMENT data into an image, which
can later be retrieved using:

docker inspect IMAGEID

Copied the test example and some description data from Erik Hollensbe
<github@hollensbe.org>

Add support to allow docker import to populate Comment field

This patch also adds an option to docker import -m  Which will allow
base image builder to populate docker images with more comments then
just imported from FOOBAR.  This matches the docker commit -m functionality.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)

Conflicts:
	api/client/commands.go
	api/server/server.go
	docs/man/docker-import.1.md