fix RUN-PROGRAM :WAIT T on Windows

fix RUN-PROGRAM :WAIT T on Windows

  Recent change to report the errno from exec() broke things on Windows.

  There when :WAIT is true it is the C code that waits, and the subsequently
  returns the exit code of the process -- meaning our attempt to use 0 to
  indicate exec() failure makes every successfull execution on Windows look
  like an exec failure.

  Oops. Use -2 instead. MORE MAGIC.