Implement the debugstatus package.

Abstract out some helper functions to be used in the charm store and similar services.

(Review request: http://reviews.vapour.ws/r/628/)
Given that the checkers themselves return their name, I wonder whether they
should return their key too, so the argument to this function would be []StatusCheckerFunc (or even better ...StatusCheckerFunc).

Then there's no risk of the keys used for the same checker being different across packages.