unix/stream: fix EVFILT_WRITE after we did shutdown(2)

When fixing bug 286692, the change eafe5967ac558, that fixed a case when
peer side does close(), also had regressed a case when our side does
shutdown(SHUT_WR). These actually are two independent code paths, and the
eafe5967ac558 shouldn't have touched the second block. The removal of
'kn->kn_flags |= EV_EOF' was incorrect and the statement on original
behavior in the commit message was also incorrect.

Do not add back so_error setting, since I failed to find a test case that
would return anything but 0 in kevent.fflags when run on stable/14.

This was found with help of https://github.com/tokio-rs/mio. Add a test
case into our test suite for that.

Fixes: eafe5967ac558de142d91660e18e9238289890e3

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D52327