Summary of changes since Inception Review:

1. Minor changes to the socket filter design; most notably the addition of
   two new callbacks (sofop_accept and sofop_mblk_prop) and having separate
   callbacks for active and passive attach.

2. The SMF service for a filter was previously
   svc:/network/socket/filter:<name>, which has been changed to
   svc:/network/socket-filter/<name>. So there is now one service per
   filter.

3. Exported interfaces has been updated (see 20q.diff).