src/utils_db_query.c: Add support for the `Column' configuration option.

src/utils_db_query.c: Add support for the `Column' configuration option.

The support has been implemented as unobtrusively as possible:

 - The `udb_result_t' type now has two new members:
   `legacy_mode' and `legacy_position'. The `legacy_mode' member is used
   to switch to a new set of functions for handling these special
   results.

 - Those ``legacy results'' use only a small subset of the members in
   `udb_result_t'. The different behavior has been put in the following
   functions:
     udb_legacy_result_finish_result
     udb_legacy_result_handle_result
     udb_legacy_result_prepare_result
     udb_legacy_result_create

 - The non-legacy counterparts to finish, handle, and prepare
   automatically call the legacy version if the `legacy_mode' member is
   set to `1'.

WARNING: Untested code, the PostgreSQL plugin still needs to be ported.