pfctl: Move ioctl abstraction functions into libpfctl
Introduce a library to wrap the pf ioctl interface.
MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29562
(cherry picked from commit 0d71f9f36e6c1849fdaf6c1edc1178be07357034)