Adding Exec method to native execdriver.
Modified Attach() method to support docker exec.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Adding support for docker exec in daemon.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Adding 'exec' command to remote API and CLI.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Import nsenter in docker.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Fix bug in attach handling for docker exec. Add docs for 'docker exec' feature.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Update cli.md to include 'docker exec' feature.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Adding integration tests for docker exec feature.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Address review comments.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Removing 'exec' feature from the CLI until the docker daemon supports resizing of
tty sessions for exec'ed commands.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Splitting the exec remote API into two separate APIs inorder to support resizing of tty sessions.
1. /container/<name>/exec - Creates a new exec command instance in the daemon and container '<name>'. Returns an unique ID for each exec command.
2. /exec/<name>/start - Starts an existing exec command instance. Removes the exec command from the daemon once it completes.

Adding /exec/<name>/resize to resize tty session of an exec command.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Adding state to exec commands to prevent multiple starts of a single exec command.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Adding docker exec support in CLI.
Fixed a bug in daemon that resulted in accessing of a closed pipe.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Merge pull request #8062 from vishh/run_in_phase2

Add support for 'docker exec' - phase 2
Add exec to autocompletions

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Merge pull request #8070 from jfrazelle/exec-autocompletions

Add exec to autocompletions