Potential 'nofork' option to allow calling IPC::Cmd::run vs. run_forked

Hello,

Thanks for writing this module.  I've found it handy.

With the addition to using IPC::Cmd::run_forked, it makes using the perl debugger more problematic.  The intention of this patch, if you choose to take it, is to default to run_forked, but provide an override for those who still want to use ::run vs. ::run_forked while debugging.

I tested both methods with a couple different error scenarios, but I reckon it could use a review.  Let me know what you think.