Add a new back-end function to return the exit code of the remote

Add a new back-end function to return the exit code of the remote
process. This is functional in SSH, and vestigial (just returns 0)
in the other three protocols. Plink's Windows exit code is now
determined by the remote process exit code, which should make it
more usable in scripting applications. Tested in both SSH1 and SSH2.

[originally from svn r1518]