MINOR: cli: filter the list of commands to the matching part

MINOR: cli: filter the list of commands to the matching part

The error message on the CLI has become unreadable due to the long list
and it's not even sorted, making it even harder to figure the right
command.

This patch starts by looking if some of the words match something known,
and if so, will limit the listing only to those commands that start like
the current one. The "help", "prompt" and "quit" commands are always
shown to help the user try something else. Now thanks to this, typing
"add" or "del" will only list "add acl", "add map" and not 50 lines
anymore.

As a small bonus, we won't print "Unknown command" anymore in response
to the "help" command.