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

pfctl: Use the new DIOCGETRULENV ioctl

Create wrapper functions to handle the parsing of the nvlist and move
that code into pfctl_ioctl.c.
At some point this should be moved into a libpfctl.

MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29560