Rewrite special file handling on Windows

This change gets GNU grep working. What caused it to not work, is it
wouldn't write to an output file descriptor when its dev/ino equaled
/dev/null's. So now we invent special dev/ino values for these files

libc/calls/writev-nt.c

@@ -27,6 +27,7 @@ textwindows ssize_t sys_writev_nt(int fd, const struct iovec *iov, int iovlen) {
   switch (g_fds.p[fd].kind) {
     case kFdFile:
     case kFdConsole:
+    case kFdDevNull:
       return sys_write_nt(fd, iov, iovlen, -1);
     case kFdSocket:
       return _weaken(sys_send_nt)(fd, iov, iovlen, 0);