kqueue: Make watcher.Close() O(n) instead of O(n^2) (#233)

kqueue: Make watcher.Close() O(n) instead of O(n^2) (#233)

In the old implementation watcher.Close() would clone the list of
watches and then run Remove() on that, and every Remove() call would
iterate over the full list of watches.

This stores the watches more efficiently so that Remove() doesn't need
to iterate over the full list: instead of merely a path → fd map, it
also stores a parent-path → list-of-files.

No functional changes, just a performance improvement.