Implement auth-token-user

Implement auth-token-user

When not using username and password (i.e. auth-user-pass) it can still
be desirable to provide the client with an auth-token, e.g. for allowing
a session to continue after a reconnect without requiring 2FA again.

However, without --auth-user-pass openvpn does not have a username and will
ignore any pushed auth-token command.

This patch adds support for auth-token-user to set the username that should
be used for auth-token

The spec of using auth-token-user base64-encoded-user are the ones that
OpenVPN3 already implements.

Patch V2: Improve style, fix comments and commit message

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20210520151148.2565578-2-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22417.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit b398aa37ca309948b481401adf0074ea5589eb2d)