Fixed: FS#2005 - Username prefix for DB user is wrong when the database is created by a reseller for the client

BugFix: FS#1234 - CLIENTNAME value when in reseller account - fixed with suggestion from Ciaran Johnston (cj). I make initial test and this worked fine but, need check if this fix impact affect any another part of ispconfig.

added the possibility to define more prefixes (implemented only for db-modul). Because a database my only be 32 chars in utf-8 it's better for my purposes to use CLIENTID instead of CLIENTNAME

- Updated the user and database prefixes to use this storage.