pfctl, libpfctl: introduce pfctl_pool
Introduce pfctl_pool to be able to extend the pool part of the pf rule
without breaking the ABI.
Reviewed by: kp
MFC after: 4 weeks
Differential Revision: https://reviews.freebsd.org/D29721
(cherry picked from commit 600bd6ce0639c84b763516477250df5964e8edf6)