RFA: Fix the shell used to execute mloop.in

[Frank Ch. Eigler]

Hi -

> [...]  Essentially though I think that the correct thing to do is to
> allow the user the choice of which shell they are going to use to
> execute scripts, precisely because of the possibility of bugs in the
> "standard" shell they might exist on their host machine.

OK, thanks.  If this is the best way of doing that (e.g., better than
environment variables), please go ahead.

- FChE