Showstopper: unix_send_process is badly broken

It appears that unix_send_process was broken by the MS Windows port. I'm getting repeated SIGPIPE crashes (which are supposed to be handled -- I have not yet reduced it to a simple test case, though I have found a repeated signal 11 test case), and the `This function can GC' labeling has been removed even though it can GC. This is a show stopper and I'm not releasing 21.0 until it is fixed. It wasn't this badly broken in 20.4. [I'm not interested in assigning blame, I only want the code fixed.]