The package currently implements the operations defined by
https://www.kernel.org/doc/Documentation/networking/filter.txt , which
comprises the base BPF virtual machine plus the Linux kernel's extension
opcodes.

Updates golang/go#14982

Change-Id: Iafb43d80e067040e60465a9bfb7d5f2ca90cc2ae
Reviewed-on: https://go-review.googlesource.com/21212Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>