DB layer enum functions are inconsistent

[ChristianBeer]

The basic enum* functions in html/inc/db_conn.inc support an argument for where and order clauses and the where clause must not start with "where". The table specific enum functions in html/inc/boinc_db.inc sometimes have one sometimes two arguments. See BoincProfile::enum and BoincCreditUser::enum.

This should be made consistent, as in: All table specific enum implementations should support two arguments. In addition a third argument for limit clauses should be added to the basic functions and the class implementations. This will make the DB layer more generic and will allow switching to something other than mysql in the future.