This commit relaxes SendmsgN behavior of introducing a dummy 1-byte
payload when sending ancillary-only messages.
The fake payload is not needed for SOCK_DGRAM type sockets, and actually
breaks interoperability with other fd-passing software (journald is one
known example). This introduces an additional check to avoid injecting
dummy payload in such case.

Backport of https://go.googlesource.com/go/+/93da0b6e66f24c4c307e0df37ceb102a33306174
Full reference at https:/golang.org/issue/6476#issue-51285243

Change-Id: I7cf00a1c7cde75fe62e00b98ccba6ac8469b0493
Reviewed-on: https://go-review.googlesource.com/60190Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>