Add generic kqueue() and kevent() support to the LinuxKPI character
devices. The implementation allows read and write filters to be
created and piggybacks on the poll() file operation to determine when
a filter should trigger. The piggyback mechanism is simply to check
for the EWOULDBLOCK or EAGAIN return code from read(), write() or
ioctl() system calls and then update the kqueue() polling state bits.
The implementation is similar to the one found in the cuse(3) module.
Refer to sys/fs/cuse/*.[ch] for more details.

MFC after: 1 week
Sponsored by: Mellanox Technologies