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.)