Implement configurable detach key

Implement configurable detach keys (for `attach`, exec`, `run` and
`start`) using the client-side configuration

- Adds a `--detach-keys` flag to `attach`, `exec`, `run` and `start`
  commands.
- Adds a new configuration field (in `~/.docker/config.json`) to
  configure the default escape keys for docker client.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

daemon/exec/exec.go

@@ -25,6 +25,7 @@ type Config struct {
 	OpenStdout    bool
 	CanRemove     bool
 	ContainerID   string
+	DetachKeys    []byte
 
 	// waitStart will be closed immediately after the exec is really started.
 	waitStart chan struct{}