OpenBSD: repair --show-gateway

OpenBSD: repair --show-gateway

OpenBSD route sockets do not want to be passed RTA_IFP on RTM_GET
- if we do this, we get back EINVAL.

On other platforms, if we do not request RTA_IFP, we will not get
back interface information for queried routes - on OpenBSD, RTA_IFP
comes back always...

So we need to #ifdef this, RTA_IFP on all platforms except OpenBSD.

(Found this fix in OpenBSD's ports tree, in their patches for OpenVPN
2.6.8 - but they just remove RTA_IFP, no #ifdef, so we can't just apply
their patch)

While at it, add M_ERRNO to the "write to routing socket" error message.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20240101094054.38869-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27892.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>