in pf_set_protostate(), only decrement the half-open states counter when

in pf_set_protostate(), only decrement the half-open states counter when
the state was created on this host, i. e. not for those pfsync-imported.
whether pfsync-imported states should be accounted is a seperate discussion,
but as things are, we only increment the counter in pf_create_state(), and
imported states don't excercise that path.
probably fixes the half-open states accounting underflow-wraparounds that
some people have been seeing.
ok sashan