spamassassin: use virtual user home directories under /var/lib/spamassassin to...

spamassassin: use virtual user home directories under /var/lib/spamassassin to avoid permission problems

When spamd isn't running as 'root', it cannot access the usual ~/.spamassassin
path where user-specific files normally reside. Instead, we use the path
/var/lib/spamassassin-<user> to store those home directories.