kqueue: don't set up watchers on unreadable files

[arp242]

Watching an entire directory would fail if it contains a file we can't
read due to permission errors. That's obviously not ideal, so just skip
the file.

Also add a test for trying to watch a directory where the permissions
are denied, and test the error is what's expected; this prevents
regressions like in #393.

Fixes #439

[arp242]

Not sure why the BSD tests are failing; for example for FreeBSD:

--- FAIL: TestWatch (0.00s)
	--- FAIL: TestWatch/file_in_directory_is_not_readable (0.98s)
		helpers_test.go:33: 
			have:
				WRITE                "/file"
				REMOVE               "/file"
				REMOVE               "/file-unreadable"
			want:
				WRITE                "/file"
				REMOVE               "/file"
--- FAIL: TestAdd (0.00s)
	--- FAIL: TestAdd/permission_denied (0.22s)
		integration_test.go:504: error is nil

But if I test locally it's fine 🤔

[~cg/fsnotify](kqueue-unreadable)% go-on freebsd go test -race
go-on: freebsd already started; nothing to do
FreeBSD 13.1-RELEASE
PASS
ok      github.com/fsnotify/fsnotify    9.030s

I think the chmod is not running properly for some reason.

[arp242]

Oh, the NetBSD failure has a clue:

Pseudo-terminal will not be allocated because stdin is not a terminal.
We recommend that you create a non-root account and use su(1) for root access.

So I guess the tests are always run as root, and root can always access everything :-/

I fixed it for FreeBSD; need to look at the others.