fusefs: fix more intermittency in the dev_fuse_poll tests

When using poll, kevent, or select there was a race window during which it
would be impossible to shut down the daemon. The problem was that poll,
kevent, and select don't return when the file descriptor gets closed (or
maybe it was that the file descriptor got closed before those syscalls were
entered?). The solution is to impose a timeout on those syscalls, and check
m_quit after they time out.

Sponsored by: The FreeBSD Foundation