Error Code Fixes

• mssql: errorCode() returns DB's code instead of SQL Server's code. Added errorNative() to get Server's code.
• Added mappings in several drivers to provide consistency between ibase, ifx, mssql, oci8, odbc(db2), pgsql, sqlite and sybase.
• If error code portability is turned on, that consistency is expanded to mysql, mysqli and odbc(access).
• Added DB_ERROR_CONSTRAINT_NOT_NULL for handling null values in NOT NULL columns.