Just like the EPOLL_CTL_ADD emulation path, the EPOLL_CTL_MOD
and EPOLL_CTL_DEL emulation paths can fail if attempting to
install an EVFILT_WRITE for a read-only pipe.
I've only observed this on the EPOLL_CTL_DEL emulation path, but
I suspect it could happen on the EPOLL_CTL_MOD path as well.
Increasing the amount of read-only pipes we rely on with altid
exports via sqlite3 made this old bug more apparent and
reproducible while looping the test suite.
This may be adjusted in the future to deal with write-only
pipes, but we currently don't have any of those watched by
kqueue.