libpfctl: introduce a handle-enabled variant of pfctl_add_rule()

libpfctl: introduce a handle-enabled variant of pfctl_add_rule()

Introduce pfctl_add_rule_h(), which takes a pfctl_handle rather than a
file descriptor (which it didn't use). This means that library users can
open the handle while they're running as root, but later drop privileges
and still add rules to pf.

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