Pull up following revision(s) (requested by thorpej in ticket #1353):

sys/miscfs/fifofs/fifo_vnops.c: revision 1.90
tests/kernel/kqueue/write/t_fifo.c: revision 1.5

- Add a new EVFILT_WRITE test case for FIFOs that correctly validates
the writability thresholds.
- Fix a bug in fifo_kqfilter() exposed by the new test case; in the
EVFILT_WRITE case, we were attaching the wrong end of the socket
pair to the knote!
- In filt_fiforead(), use ">= so->so_rcv.sb_lowat" rather than "> 0"
for consistency with fifo_poll().  NFC.

(martin)