Pass command exit status via sideband to client.

Pass command exit status via sideband to client.

Currently the only way for the client side to tell whether a command
failed is to parse the command output. This patch changes command.c to
send an empty sideband packet at command termination. If the command
succeeded, the sideband designator is set to SBD_EXIT__SUCCESS,
otherwise it is SBD_EXIT__FAILURE.

The client checks for these sideband packets and terminates with
EXIT_FAILURE if it received the SBD_EXIT__FAILURE packet.