Add "make validate" for both local and CI gofmt and DCO verification

With this PR, it becomes as simple as `make validate` (or `make all`, of course) to verify locally that your commits conform to the DCO and are properly `gofmt`'d.

Also, as a side benefit, we now handle half of the DCO exemption programmatically ("Your patch is a single line change to documentation."), and the "validate-dco" script will not fail if your entire PR is no more than one line added and/or one line removed from the docs directory.

The foundation of this was my hack-day project at the birthday party, and I just barely got a chance to finish fleshing it out this evening. :)