Refactor status to be a type

[martialblog]

For the v1 Release we want to introduce a proper State Type for the v1.0.0 - that type can then just implement the Stringer interface and have a NewFromInt/String function.

Fixes #125

This also changes check.ExitRaw to Exit and removes the Exitf.

[martialblog]

Not sure about this one. I kinda wanted something to "mean" error/invalid and we use a -1 value in result.WorstState.

[RincewindsHat]

That is one problem of the whole monitoring plugins interface, there is no "error" state

[martialblog]

Any ideas? just use Unknown and add some custom error types?

[RincewindsHat]

hm, we could keep an "error" code and map it to "UNKNOWN" at exit, that would be nice from a programer perspective, but also confusing since the end is the same.

[RincewindsHat]

hm, or keep the Status pure and force everyone to use a wrapper with an error.

[RincewindsHat]

Not ideal, but there is only so much one can do under the given circumstances.
In general I think this is an improvement and we should go with it.

[RincewindsHat]

sorry @martialblog, I originally wanted to just fix the linter things, but it got out of hand.
I did remove the Invalid status in the end, since I think it should not be there at all, specifically not publicly.
The Statuses are there to symbolize certain test results and I tend to prefer to keep them limited to the "official" ones.

In that spirit I also removed the residual relations between Status and int (namely NewStatusFromInt) to discourage people from using the interchangeable.

If all of that was a stupid idea, don't hesitate to revert those commits.