[Serusers] Auth* changes

I just committed some changes into 0.9.0 auth* modules - basically it's just a backport from the development branch. Mainly to solve some issues regarding the RPID handling (there were quite some reports about RPID problems in 0.9.0), which was one of the last drawbacks before 0.9.0 release. Interface changes: auth_db: param "load_credentials": list of '|' separated columns to be load from subscriber table at auth time. The values for each column is loaded in a corresponding AVP (string type) with same name as the column; Default value is "rpid". Empty value "" disable this feature. auth: param "rpid_avp": avp name (string) which carry the RPID value - must be synchronized with auth_db if RPID is fetched from DB. Default value is "rpid". Please all 0.9.0 users, give it a try a report any potential problems. bogdan