libpfct: factor out pfctl_get_rules_info()

Introduce pfctl_get_rules_info(), similar to pfctl_get_eth_rules_info()
to retrieve rules information (ticket and total number of rules).

Use the new function in pfctl.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D34443