Forward the exit status from the remote command

Please change the mssh command to forward the exit status of the remote command the same way ssh does.

The man page from ssh describing this:
> EXIT STATUS
>      ssh exits with the exit status of the remote command or with 255 if an error occurred.

Example:
```
kim@splint:~$ mssh ec2-user@i-012345678990 exit 42 ; echo $?
0
kim@splint:~$ 
```
This should echo `42` instead of `0`.
