null: support EVFILT_READ/EVFILT_WRITE kevent filters

This enhances the full, null, and zero devices to support read and write kevent filters.

A read event is immediately triggered for full and null, and never for zero.
A write event is immediately triggered for zero and null, and never for full.

Reviewed by: cognet
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51807