pf: Do not flush on reload

pfctl only takes the last '-F' argument into account, so this never did what
was intended.

Moreover, there is no reason to flush rules before reloading, because pf keeps
track of the rule which created a given state. That means that existing
connections will keep being processed according to the rule which originally
created them. Simply reloading the (new) rules suffices. The new rules will
apply to new connections.

PR: 127814
Submitted by: Andreas Longwitz <longwitz at incore.de>
MFC after: 3 weeks