New database object classes

Pair of objects which supported by Interbase/Firebird/Red Database:

db_exception - exception which can be thrown from PSQL
db_domain - named set of column attributes
Fixes for db_domain and db_exception

Rename db_domain to db_type
Add "use" permission to db_domain and db_type
Renamed db_type to db_datatype, to avoid confusion with SELinux "type"
Merge pull request #1 from artyom-smirnov/master

New database object classes for database exceptions (e.g. Firebird Procedural SQL) and database datatypes.