passfd module, part 2, tweaks.

passfd module, part 2, tweaks.

* modules/passfd (Files): Reorder.
(Depends-on): Remove errno.
(Include): Remove <sys/socket.h>, <sys/un.h>.
* lib/passfd.h: Use a GPLv3+ header. Make C++ safe.
* lib/passfd.c: Untabify. Use a GPLv3+ header. Really include the
specification header. Include <sys/socket.h> always. Don't include
<winsock2.h>. Use "#if HAVE_..." instead of "#ifdef HAVE_...".
(sendfd): Clarify that it sets errno when it fails.
(recvfd): Fix specification.