libpfctl: retrieve family id only once

Look up the pfctl family id when we open the handle, rather than for
every function call.
This saves us a lot of netlink calls, at the expense of storing one
extra int in the handle.

Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 2a478dfc7f9cd60037939e121026bf26a01e8c41)