Fix spamassassin symlink.

At some point in the distant past there was a config file called
spam.assassin.prefs.conf. This was renamed to spamassassin.conf,
but the symlink in %D/etc/mail/spamassassin still pointed to the
old location. This worked fine for old installs, but new installs
were left out. This should DTRT for both without administrator
intervention necessary.