Improve and modernize the SQL system API.

Improve and modernize the SQL system API.

- Move everything into the SQL namespace and drop the SQL prefix.
- Move SQLProvider::PopulateUserInfo to SQL::PopulateUserInfo.
- Rename SQLEntry to SQL::Field and clean up.
- Rename SQLEntries to SQL::Row.
- Rename SQLerror to SQL::Error and clean up.
- Rename SQLerrorNum to SQL::ErrorCode and drop the SQL_ prefix.
- Rename ParamL to SQL::ParamList.
- Rename ParamM to SQL::ParamMap;
- Make implementing SQLQuery::OnError mandatory.
- Redo most of the documentation in the sql header.