The copyright and license notice is already in the LICENSE file. There
is no justifiable reason to also require that it be included in every
file, since the individual files are not individually distributed except
as part of the entire package.
A child process created with .fork() needed to call `process.exit()` explicitly
because the communication channel with the parent kept the event loop alive.
Fix that by only ref'ing the channel when there are 'message' event listeners.
Fixes#3799.