fs: make unwatchFile() remove a specific listener

fs: make unwatchFile() remove a specific listener

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.