used when a read filter is requested on a write-only fifo descriptor, or
a write filter is requested on a read-only fifo descriptor. This
permits the filters to be registered, but never raises the event, which
causes kqueue behavior for fifos to more closely match similar semantics
for poll and select, which permit testing for the condition even though
the condition will never be raised, and is consistent with POSIX's notion
that a fifo has identical semantics to a one-way IPC channel created
using pipe() on most operating systems.
The fifo regression test suite can now run to completion on HEAD without
errors.
MFC after: 3 days
88f39e8e95