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

(cherry picked from commit 0d6c8174ef2f3d8d6fb02ec97753a0937796a5c6)