tun.c: enable using wintun driver under SYSTEM

tun.c: enable using wintun driver under SYSTEM

Commit 6d19775a468 has removed SYSTEM elevation hack,
but introduced regression - inability to use wintun without
interactive service.

Proceed with ring buffers registration even if iservice is unavailable
and display relevant error message.

Trac: #1318

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20200819070746.197-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20780.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>