JENKINS-5931 want to restrict some user to access the commands, and JENKINS-11606 disable the commands in channel room.
But ircbot still allow unauthorized access by private chat.
(The "/msg ircbot ircbot: help" or "/msg ircbot !prefixCommand help" messages can control ircbot.)

After added "disallow private chat commands" option, we can control the usage of commands to a specific group of users.
ex:
> Channels:
> - channel-notification / no password / enable "Notification only"
> - channel-admin / has password / disable "Notification only"
> Enable "Disallow Private Chat"

Unauthorized users join channel-notification to get irc notifications, the authorized users join channel-admin to control the jenkins by commands.
And other unauthorized users cannot control jenkins by private chat.