Allow sending SIGINT to child processes on Windows *if* they are running Perl

In Perl 5.6 it was possible to send SIGINT to child
processes on
Windows. One problem was that the signal would be delivered
to *all*
child processes in the same process group.

Perl 5.8 has some initial support for sending Console
Control Events to
child processes by starting each of them as a new process
group:

    http://public.activestate.com/cgi-bin/perlbrowse/p/14163


It seems like this code only works for SIGBREAK. Using the
CREATE_NEW_PROCESS_GROUP flag disables SIGINT signals sent
by
GenerateConsoleCtrlEvent() by default (this does not seem to
affect
Ctrl-C events generated by the keyboard).

The attached patch re-enables sending SIGINT to console
child processes
if the child is running Perl and the system is Windows NT or
later. This
doesn't sound that useful, but it is the best I could come
up with.

Cheers,
-Jan