Make sure that pf_step_into_anchor always saves a pointer to the rule

Make sure that pf_step_into_anchor always saves a pointer to the rule
that owns the anchor on the pf anchor stack.  There's no reason why we
should check for depth here.  As a side effect this makes sure that the
correct nested anchor gets it's counter bumped instead of the top most.

For the save/restore symmetry pf_step_out_of_anchor is made to always
restore previous value of the anchor rule.  depth == 0 means what we a
at the top (main ruleset).

OK henning