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.
Before this commit, `fs.unwatchFile(path)` removed *all* listeners for `path`.
The function is overloaded now: `fs.unwatchFile(path)` still removes all
listeners, but `fs.unwatchFile(path, cb)` lets you remove a specific listener.
Fixes #3660.
