RADIUS: T5577: Added mandatory and optional modes for RADIUS

In CLI we can choose authentication logic:

• mandatory - if RADIUS answered with Access-Reject, authentication must be stopped and access denied immediately.
• optional (default) - if RADIUS answers with Access-Reject, authentication continues using the next module.

In mandatory mode authentication will be stopped only if RADIUS clearly
answered that access should be denied (no user in RADIUS database, wrong
password, etc.). If RADIUS is not available or other errors happen, it will be
skipped and authentication will continue with the next module, like in
optional mode.