Fix TUNSETGROUP compatibility with very old Linux systems.

Fix TUNSETGROUP compatibility with very old Linux systems.

Our code works on "very old Linux" (Fedora-1), but needs a #define
for TUNSETGROUP to compile.  Everything else is there.

While at it, fix TUNSETGROUP error message.

Reported-By: noloader on Trac
Trac: #1152

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20200909153725.1158-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20932.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>