Make management password check constant time

Make management password check constant time

This changes the password check on the management interface to be constant
time. Normally the management port should not be exposed in a way that
allows an attacker to even interact with it but making the check constant
time as an additional layer of security is always good.

Patch v2: include NUL byte in comparison

Reported-by: Connor Edwards <cedw@pm.me>
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221220140458.2666637-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25784.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>