Currently Darwin and FreeBSD support and NetBSD and OpenBSD do not
support EV_RECEIPT flag. We will drop use of EV_RECEIPT for now.

Also enables to build runtime-integrated network pollster on
freebsd/amd64,386 and openbsd/amd64,386. It just does build but never
runs pollster stuff.

This is in preparation for runtime-integrated network pollster for BSD
variants.

Update #5199

R=dvyukov, minux.ma
CC=golang-dev
https://golang.org/cl/11759044