Introduce new make targets to check and add license headers to files
("make lint-license" and "make fix-license"). License checking is now a part of
"make lint" as well.
Initial attempts used goheader, but it caused "make lint-go" to become about 9x
slower (if not more), plus it only targets go files. Google's addlicense is fast
enough and targets however many file types we want.
Existing files that were missing licenses got the header added, using the
current year as the license date.
* Introduce lint-license and fix-license make targets
* Ignore generated files
* Add license to go files
* Replace irregular licenses with standard copyright/license
* Add license to proto files
* Install addlicense in build.assets Dockerfile
Jakub Nyckowski