runtime: use EVFILT_USER to wake up kevent where available

[panjf2000]

OSX 10.6, FreeBSD 8.1, DragonFlyBSD 4.0, and NetBSD 10.0 added EVFILT_USER for user-established events. By utilizing EVFILT_USER, we'd be able to eliminate the kernel overhead of the pipe and gain a worthwhile performance boost for the notification mechanism of kqueue-based netpoller.

See also #65443

@golang/runtime

[gopherbot]

Change https://go.dev/cl/577895 mentions this issue: runtime: utilize EVFILT_USER to wake up kevent for kqueue