kqueue: fix removing a directory

kqueue: fix removing a directory

Fix regression from #526, which would sometimes fail with something
along the lines of:

    --- FAIL: TestWatchRm/remove_watched_directory (1.30s)
        helpers_test.go:384: fsnotify.sendDirectoryChangeEvents: open /tmp/TestWatchRmremove_watched_directory2055111636/001: no such file or directory
        fsnotify_test.go:750:
            have:
            	REMOVE               "/a"
            	REMOVE               "/b"
            	REMOVE               "/c"
            	REMOVE               "/d"
            	REMOVE               "/e"
            	REMOVE               "/f"
            	REMOVE               "/g"
            want:
            	REMOVE               "/"
            	REMOVE               "/a"
            	REMOVE               "/b"
            	REMOVE               "/c"
            	REMOVE               "/d"
            	REMOVE               "/e"
            	REMOVE               "/f"
            	REMOVE               "/g"
            	REMOVE               "/h"
            	REMOVE               "/i"
            	REMOVE               "/j"

We can just always ignore a directory no longer existing; kqueue should
still send the correct events.

Also noticed the error on sendFileCreatedEventIfNew() wasn't really
being sent properly, so deal with that as well.