rsh.exp backward compatibility patch

[DJ Delorie]

Does this make sense?  We're trying to make the new dejagnu compatible
with our old board library (which still has to support older dejagnu
versions), which doesn't have the (now mandatory) extra arguments.

diff -p -U3 -r1.4 rsh.exp
--- lib/rsh.exp 4 Aug 2006 10:44:50 -0000       1.4
+++ lib/rsh.exp 26 Sep 2007 20:32:37 -0000
@@ -223,7 +223,7 @@ proc rsh_upload {desthost srcfile destfi
 
 # Execute CMD on BOARDNAME.
 #
-proc rsh_exec { boardname program pargs inp outp } {
+proc rsh_exec { boardname program pargs {inp ""} {outp ""} } {
     global timeout
 
     verbose "Executing $boardname:$program $pargs < $inp"