Add log rotation of spamd.log, also option to increase log retention

[Haw Loeung]

Hi,

We're missing log rotation of spamd.log. Something like this to /etc/logrotate.d/spamd:

| /var/log/spamd.log
| {
| rotate 26
| weekly
| missingok
| copytruncate
| notifempty
| compress
| delaycompress
| }

Also, would be nice to have the option to increase log retention for both spamd.log and mail.{log,err}. The default of 4 weeks is too short.