pf: initial SCTP support

Basic state tracking for SCTP. This means we scan through the packet to
identify the different chunks (so we can identify state changes).

MFC after: 3 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D40862

(cherry picked from commit 010ee43f5673eea4c86f846893eadc3c5529b2f8)