Only set the terminal in raw mode for commands which need it

The raw mode is actually only needed when you attach to a container.
Having it enabled all the time can be a pain, e.g: if docker crashes
your terminal will end up in a broken state.

Since we are currently missing a real API for the docker daemon to
negotiate this kind of options, this changeset actually enable the raw
mode on the login (because it outputs a password), run and attach
commands.

This "optional raw mode" is implemented by passing a more complicated
interface than io.Writer as the stdout argument of each command. This
interface (DockerConn) exposes a method which allows the command to set
the terminal in raw mode or not.

Finally, the code added by this changeset will be deprecated by a real
API for the docker daemon.
Simplification in the goroutine that restore the terminal state on SIGINT