[MERGE] Add msgeditor hooks to known_hooks.

[James Westby]

This means that the testsuite prevents any registered hooks from
triggering during the testsuite.
  
It also adds comment to try and give the meaning of the parameters.
As it took me a few minutes to figure out what each was for,
why there was duplication for most, and why one pair is a tuple and
the other isn't (though I'm still not entirely sure).

Thanks,

James