Give auth_brute ability to try credentials stored in db

This change will enable all the modules deriving from auth_brute to
try the credentials stored in the database.

The purpose is to find users utilizing the same password for other services.

I added two options:
DB_USERPASS: this will try each user/pass couple stored in the db

DB_ALL_USERS: this will add every known username from the DB to the user list

DB_ALL_PASS: this will add every known password from the DB to the password list.