wintun: remove SYSTEM elevation hack

wintun: remove SYSTEM elevation hack

As discussed a while ago on the mailing list and
community meetings, having SYSTEM elevation hack
inside openvpn code considered harmful.

Since interactive service is the recommended way
of using openvpn on Windows, limiting wintun usage to
interactive service should not be an issue.

Remove elevation hack code and provide an error message
telling user to use interactive service or do SYSTEM
elevation himself via psexec.

Move implementation of register_ring_buffers() to header
amd delete ring_buffer.c.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200724104841.89-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20567.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>