Fix spamassassіn policybank handling
There was a problem with spamassassin and policybank that reached to the effect
that some policybank settings got applied for other policybanks during the
lifetime of an amavisd-new child. This patch backs up the config before
launching the policybank rules and restores them after that, so that the
spamassassin object does not get polluted by other policybanks.