pmctl watchdog tests

For https://github.com/strongloop-internal/scrum-nodeops/issues/219

Add some tests.
Done. I also reorganized more of the text in 880b27b, what do you think?

I know that simpler utilities, like pm-install, can fit all the description of their options into a single line, or sometimes with just one extra half-screen width, but I think that the more complex utilities like pmctl are getting more text in their usage, and that that model isn't working so well, and that it looks better like this.

Also, pm-install (for example) might benefit soon (once people start using it) from more inline docs in the usage, so this form might become more common.

What do you think?