Add entries for unused instruction encodings.

Most are reserved; two are used by Linux for mod and XOR instructions,
and two are used by NetBSD for "coprocessor" operations.
Check whether the compiled filter passes validation.
Support mod and XOR operators.

Same opcodes as on Linux.  We support them in BPF filters, but warn, in
the man page, that using them on anything other than Linux 3.7 or later
will cause the filter to run in userland and thus require more resources
and perhaps cause more packets to be dropped.  (The filter will
presumably be rejected by the kernel-mode code if it doesn't support
BPF_MOD or BPF_XOR, and libpcap will fall back on running the filter in
userland.)
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.