[Rd] system on windows vs. unix

[Gabor Grothendieck]

The R system command has different arguments on Windows and UNIX.
I hadn't realized that and I think it would be nice if the input=
argument available
on Windows were available on UNIX too and the ignore.stderr= argument
available on
UNIX were avaliable on Windows too.  

Even without that I could have saved some time if the help file had
pointed out that
the arguments vary from OS to OS or even better which are common and which 
are OS-specific.