make 'set skip on ...' in pf.conf dynamic

make 'set skip on ...' in pf.conf dynamic

This is an old issue in pf(4): whenever new interface appears
in IP stack, we must reload pf.conf to apply 'set skip on ...'
to newly plumbed network interfaces. Time has come to fix it.
The idea is to also create pfi_kif for interfaces, which are
referred by 'set skip on ...'. Such pfi_kif instances are
created/destroyed by pfi_set_flags()/pfi_clear_flags().

claudio@ dragged my attention to this in Gouveia. Also his
feedback helped me to put change into shape.

OK claudio@