Clean up old schema updater.
Shift configuration files from Code/ to conf/.

* options.inc renamed to options.php (by default).
* Look for options in (1) conf/options.php, (2) conf/options.inc,
  (3) Code/options.inc.
* Move distoptions.inc into src/distoptions.php.
Database programs accept `-c CONFIGFILE` options.

Allows us to run multiple configurations out of the same directory.
createdb: Error message format.
Internal: Refactor $Opt["dsn"] setting.
Nit.
Use Conference::save_setting when possible.
Internal: Create conferenceKey in Conference.
Conference nits.
External code might have already loaded $Conf and $Opt.
Remove unused tableMessages.
Mark variables as private.
Settings invariants are all enforced in load_settings.
Begin test framework.

The goal is to be able to run CLI tests with `php test/test.php`.