github: Enable github workflow output from checkstyle9.pl

Let's see if we can get the style issues flagged inline.

Sponsored by: Netflix

GitHub: style: Add the synchronize activity type

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1064

style.yml: Don't run this on branch pushes

We don't need to run this on branch pushes, just pull requests. It's
designed to be a gross filter for incoming commits, not something
perfect we need to keep green. It also doesn't work quite right for
branch pushes anyway and needs adjustment.

Also remove some debugging information. We don't need it anymore.

Noticed by: jrtc27
Sponsored by: Netflix

Connect my checkstyle9.pl script to a action.

Connect the checkstyle9.pl script to a github action. This will provide
feedback to people submitting changes when the style is grossly wrong. And
can provide other automated feedback for the commit message in the future.
It already catches the github noreply author.

It pulls the full repo to do this. Optimizations welcome. After messing
with that for a few hours, I decided to punt and commit the slow, working
version and let someone else optimize from here.

Sponsored by: Netflix