[Rd] Rterm in a Cygwin shell (in Windows) (PR#7287)

[murdoch at stats.uwo.ca]

This bug report is mostly a note to myself to track this down:

In the version of the Cygwin bash shell I'm currently using, Rterm
does not detect that it is being run interactively, so it switches
into batch mode, and lots of things stop working.

>$ bash --version
>GNU bash, version 2.05b.0(1)-release (i686-pc-cygwin)
>Copyright (C) 2002 Free Software Foundation, Inc.

Duncan Murdoch