pf: add 'allow-related' to always allow SCTP multihome extra connections

Allow users to choose to allow permitted SCTP connections to set up additional
multihomed connections regardless of the ruleset. That is, allow an already
established connection to set up flows that would otherwise be disallowed.

In case of if-bound connections we initially set the extra associations to
be floating, because we don't know what path they'll be taking when they're
created. Once we see the first traffic we can bind them.

MFC after: 2 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D48453

(cherry picked from commit e4f2733df8c9d2fd0c5e8fdc8bec002bf39811f3)