resumablerequestreader: allow initial response

Make it possible to inspect an initial response and pass it to
ResumableRequestReader. This makes it possible to inspect an initial
response and passing it to ResumableRequestReader to avoid making an
extra request.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
get layer: remove HEAD req & pass down response

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
integcli: fix TestInspectLinksStopped with Go1.3

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
pkg/units: Better to not use `error` as var name

Better to not use `error` as var name (might eclipse the error type) for
clarity and to prevent subtle bugs.

Docker-DCO-1.1-Signed-off-by: Francisco Carriedo <fcarriedo@gmail.com> (github: fcarriedo)
user facing documentation changes in master atm

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Fixed parent-child relationship description

"db" is the parent image, and "web" links to it as the child image. There was a typo in the document which reversed the relation.
Merge pull request #7176 from SvenDowideit/auto-update-master-cli-docs

user facing documentation changes in master atm
Merge pull request #7180 from rhuss/patch-1

Fixed parent-child relationship description
Merge pull request #7164 from fcarriedo/not-error-as-var-name

pkg/units: Better to not use `error` as var name
Change version to 1.1.2-dev

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #7190 from vieux/merge_release_v1.1.2

Change version to 1.1.2-dev
Merge pull request #7170 from unclejack/fix_testinspectlinksstopped

integcli: fix TestInspectLinksStopped with Go1.3
Merge pull request #6727 from unclejack/improve_bytes_range

Improve layer downloading