fifo_kqfilter() VOP implementations, since they in theory are used
only on open file descriptors, in which case the ioctls are via
fifo_ioctl_f() and kqueue requests are via fifo_kqfilter_f().
Generate warnings if they are entered for now. These printf()
calls should become panic() calls.
Annotate and re-implement fifo_ioctl_f(): don't arbitrarily
forward ioctls to the socket layer, only forward the ones we
explicitly support for fifos. In the case of FIONREAD, don't
forward the request to the write socket on a read-write fifo, or
the read result is overwritten. Annotate a nasty case for the
undefined POSIX O_RDWR on fifos, in which failure of the second
ioctl will result in the socket pair being in an inconsistent
state.
Assert copyright as I find myself rewriting non-trivial parts of
fifofs.
MFC after: 3 days
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