The remote.Cmd struct could not convey any transport related error to
the caller, meaning that interrupted commands would show that they
succeeded.
Change Cmd.SetExited to accept an exit status, as well as an error to
store for the caller. Make the status and error fields internal,
require serialized access through the getter methods.
Users of remote.Cmd should not check both Cmd.Err() and Cmd.ExitStatus()
until after Wait returns.
Require communicators to call Cmd.Init before executing the command.
This will indicate incorrect usage of the remote.Cmd by causing a panic
in SetExitStatus.
