Disable unforked interval runs.

:construction: WIP

@lamont-granquist Right track so far?
that might be TMI.

most people just won't need to know what --no-fork is or does.  people who use daemonized chef-client runs that loop with an --interval or --daemonize should just know they can no longer use --no-fork, and people running chef-client out of a cronjob should be pushed towards --once.  and the latter you can mention that they could use --no-fork if they like but they probably shouldn't.  the complexity of the interaction between --interval/--daemonize and --once is stuff nobody should really need to know.