Ask that people inform us of additions to BPF.

At minimum, we'd like to keep track of new instructions added, so that
others don't use the same encoding for different operations.  Ideally,
we'd like to update our implementation to support the new instructions
and encourage other implementations to add them as well.

Also remove the "from Linux" note on BPF_MOD and BPF_XOR, as the Linux
kernel BPF implementation isn't the only one that has them now, given
that we have them.
For historical reasons, note BSD/OS's use of some values.

See

   https://www.usenix.org/legacy/events/bsdcon02/full_papers/lidl/lidl_html/index.html

for some information on that; we probably won't be implementing those
extensions, but we might want to avoid using the encodings used by them.
Add support for filters testing for 802.2 LLC frame types.

"llc" can now be used to check for frames with 802.2 headers on
linktypes other than Sun ATM, and can now be given an argument to check
for I, S, and U frames and for particular types of S and U frames.