interactive.c: Improve access control for gui<->service pipe

interactive.c: Improve access control for gui<->service pipe

At the moment everyone but anonymous are permitted
to create a pipe with the same name as interactive service creates,
which makes it possible for malicious process with SeImpersonatePrivilege
impersonate as local user.

This hardens the security of the pipe, making it possible only for
processes running as SYSTEM (such as interactive service) create the
pipe with the same name.

While on it, replace EXPLICIT_ACCESS structures with SDDL string.

CVE: 2024-4877

Change-Id: I35e783b79a332d247606e05a39e41b4d35d39b5d
Reported by: Zeze with TeamT5 <zeze7w@gmail.com>
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20240619144629.1718-2-lev@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28808.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>