dspam

Add option to allow DSPAM to learn a whitelisted message as ham.

- Message would be whitelisted in DSPAM for a user A (because the sender of
the message is whitelisted)
- User A or a global option is set to learn whitelisted messages as ham if
the message would be normally classified as spam

Ideal would be if such a option could be turned on/off on user level and on
a global level (in dspam.conf), where the user set value would have
stronger weight then the one configured in dspam.conf.