(Spotted by Dan Kegel with valgrind).

    
This shouldn't be needed anymore now that exceptions are not handled
on the signal stack, and signal mask handling is broken on Mac OS.

The canonical way to detect EOF on read is when you request more than
zero bytes, but zero bytes come back.  Switching to this method seems
to handle a few problem cases better.  Also handle ERROR_BROKEN_PIPE
properly just in case.