Prevent Tests from creating users on Prod Index

The integration tests had previously used the environment variable
DOCKER_INDEX_URL but it was apparently removed several months ago.

Change the integration auth tests to specify the ServerAddress field
of the AuthConfig struct to use the staging deployment of the index.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Fixes #3879 by updating embedding of ascii.io (now asciinema.io)
Docker-DCO-1.1-Signed-off-by: Andy Rothfusz <github@developersupport.net> (github: metalivedev)
Merge pull request #3832 from jlhawn/3831-auth-tests-server-address

Prevent Tests from creating users on Prod Index
docker: detect defaultNetworkMtu from default route

Docker-DCO-1.1-Signed-off-by: Johan Euphrosine <proppy@google.com> (github: google)
Remove reference to <linux/btrfs.h>, and instead use <btrfs/ioctl.h> like we're supposed to (from btrfs-progs)

This fixes compilation issues when btrfs.h isn't available (because we just need the relevant structs, which for userspace programs are supposed to come from btrfs-progs instead of the kernel headers).

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Update PACKAGERS.md to explicitly talk about stripping, quoting Dave Cheney from Golang upstream (who is very, very adamant about not stripping Golang compiled binaries)

While there, remove the now-outdated section about `CGO_ENABLED=0`

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Fix docker commit output

The id is returned as Id, not ID, so print the right thing.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #3899 from tianon/do-not-strip-binaries

Update PACKAGERS.md to explicitly talk about stripping
Fix several btrfs driver error messages

Several of the error messages were wrong, but not one of them was helpful, so this fixes them to include the important information (the actual error), and a correct description of which operation failed.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3892 from tianon/remove-btrfs-header-reference

Remove reference to <linux/btrfs.h>, and instead use <btrfs/ioctl.h> like we're supposed to (from btrfs-progs)
Merge pull request #3893 from tianon/clean-up-btrfs-error-messages

Fix several btrfs driver error messages
Merge pull request #3904 from alexlarsson/fix-docker-commit-output

Fix docker commit output
Remove stackbrew prefix on ubuntu images now that they're reasonably up-to-date and stable

Everyone probably wants to run `docker pull ubuntu` before building with this change.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3906 from tianon/unprefix-ubuntu

Remove stackbrew prefix on ubuntu images now that they're reasonably up-to-date and stable
Fix login prompt on push and pull because of error message
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Fix comparing binds and volume ids from other containers

Currently comparing volume ids for binds and other containers are broken
Fixes #3749
Fixes #3885
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #3911 from crosbymichael/fix-remove-volume

Fix comparing binds and volume ids from other containers
Merge pull request #3908 from crosbymichael/fix-login-prompt

Fix login prompt on push and pull because of error message
networkdriver: add ErrNoDefaultRoute

Docker-DCO-1.1-Signed-off-by: Johan Euphrosine <proppy@google.com> (github: google)
docker/config: update -mtu flag default

Docker-DCO-1.1-Signed-off-by: Johan Euphrosine <proppy@google.com> (github: google)