libpfctl: Switch to pfctl_rule

Stop using the kernel's struct pf_rule, switch to libpfctl's pfctl_rule.
Now that we use nvlists to communicate with the kernel these structures
can be fully decoupled.

Reviewed by: glebius
MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29644