filemon: Reject FILEMON_SET_FD commands when the fd is a kqueue

When FILEMON_SET_FD is used, the filemon handle effectively wraps the
passed file. In particular, the handle may be inherited by a child
process, or transferred over a unix domain socket, so we must verify
that the backing file permits this.

Reported by: syzbot+36e6be9e02735fe66ca8@syzkaller.appspotmail.com
Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34128