BUG/MINOR: cli: make sure "help", "prompt", "quit" are enabled at master level

BUG/MINOR: cli: make sure "help", "prompt", "quit" are enabled at master level

These 3 commands are functionally valid both in master and worker CLIs.
However, while they do have a valid handler, they are not permitted by
the code and work partially by chance in the master:
  - "prompt" and "quit" are intercepted by the request analyser
  - "help" triggers an error, which results in displaying the error
    message

Let's make sure they are permitted so that we don't count errors there and
that we can report appropriate help.

This bug has always been there but it doesn't have any functional effect
at the moment since "help" can only show the error message. As such, there
is no need to backport it.