Merge modified --with-rsh patch: we now determine the default

Merge modified --with-rsh patch: we now determine the default
remote-execution command as follows:

 1) if --with-rsh is specified, use that.

 2) otherwise if remsh is in the path, use that.

 3) otherwise use rsh

If remsh is present, we always modify the order of parameters to suit
it.  This is a bit strange.