spamassassin: Run as user "spamd" instead of root.

spamassassin: Run as user "spamd" instead of root.

When running as root, spamd will run as whatever user the client claims
to be, to read preferences. Since this is not-so-secure, we'll just run
as an unprivileged user to begin with (since we don't do per-user
privileges anyway).