Force combinationation of --socks-proxy and --proto UDP to use IPv4.

Force combinationation of --socks-proxy and --proto UDP to use IPv4.

Our current socks.c code does not handle IPv6 + UDP mode (socket
negotiated with server is IPv4-only, addresses passed in the
packets are IPv4-only).  If this combination is specified, print
an explanatory message and force IPv4-only.

While at it, extend socks.c code to print address+port of auxiliary
UDP connection to SOCKS server (helps debugging).

Trac: #1221

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20191020150039.21516-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18952.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 57623b4e40e68de983b9dfd82914dadcbe096f7b)