Correctly set EVFILT_WRITE for nbio emitters when using kqueue.

Summary: Fixes a typo in corelib/nbio/kqueue.c:ph_nbio_emitter_apply_io_mask
that incorrectly set EVFILT_READ when the mask parameter specified interest in
write events.
facepalm
Good catch :-)  Was that the root cause of the issue we discussed on IRC?