libpfctl: fix file descriptor leak

pfctl_get_rules_info() opened a netlink socket, but failed to close it again.
Fix this by factoring out the netlink-based function into a _h variant that
takes struct pfctl_handle, and implement pfctl_get_rules_info() based on that,
remembering to close the fd.

While here migrate all in-tree consumers to the _h variant.

MFC after: 3 days
Sponsored by: Rubicon Communications, LLC ("Netgate")