bpfc: filter: add poff ancillary operation

In preparation for [1], add the #poff operation, so that users can
dynamically truncate packets on the payload start offset. This is
useful for several reasons: privacy, speed, ...

Example for bpfc:

  ld #poff
  ret a

  [1] http://thread.gmane.org/gmane.linux.network/262512

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>