socks: Replace magic "10" for socks header with macro

socks: Replace magic "10" for socks header with macro

So that it is easier to check that we indeed
have reserved this prior to assuming we have.

Github: openvpn-private-issues#4
Change-Id: I0aca7e7d9aa190541f11745cf72193cb6b39540a
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1467
Message-Id: <20260112171122.3994-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg35214.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>