Fix error message about extended errors for IPv4-only sockets.

Fix error message about extended errors for IPv4-only sockets.

The new code to enable IPv6 extended error reporting will cause
an error ("Protocol not available (errno=92)") if trying to enable
that setsockopt() option on an IPv4-only socket.

Fix: pass sock->info.af to set_sock_extended_error_passing(), only
apply to AF_INET6 sockets.

To make that work, ensure that sock->info.af is set to not only
the value coming from config (which might be AF_UNSPEC) but to the
actual value used in socket creation (credits: Arne Schwabe).

Add comments to make explicit that the asymmetry here (IPv4 extended
socket error reporting is enabled on all sockets) is intentional.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20220723121909.21943-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24731.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>