Fix types in WIN32 socket_listen_accept()

Fix types in WIN32 socket_listen_accept()

SOCKET_UNDEFINED is of type socket_descriptor_t (or SOCKET, in MS types),
so new_sd should be too.  Also, the return value of this function is
always stored in a socket_descriptor_t variable, so it should return that
type (which makes sense now, because it returns new_sd) instead of an int.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1489056455-6004-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14239.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 33e1a869fc6edb6bce5816b11dbecfaca57b20d4)