iservice: validate config path better

iservice: validate config path better

Instead of just rejecting any path that contains ".." use some WIN32 API
functions to combine, canonicalize and then check if the resulting
path is located under the config directory. Makes the code prettier
and more correct.

Change-Id: I0e94068f467f2899daf133b032a785d2d7fc05e4
Signed-off-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1307
Message-Id: <20251112092244.22764-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg34336.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>