[Meta] Does CiviCRM have a single defined application HTTP entry point which routes all requests?

Various issues arise when we have multiple entry points to the application (eg scripts in extern/, direct call scripts like KCFinder integration/civicrm.php, IPN handlers).

These issues stem from duplicated / non-identical approaches to bootstrapping / path discovery / session management.

The end goal here is to eliminate alternate entry points and ensure any callback uses CiviCRM's core routing.

As much as possible auth/bootstrap/url/path resolution should be handled in one place only (each) for maintainability.

Background issues / examples:

• CRM-17212
• CRM-13249
• CRM-12246
• #6 (moved)
• #7 (moved)

Targets

• extern/authorizeIPN.php, ipn.php, pxIPN.php - IPN handlers → use civicrm/ipn/% entry point
• extern/cxn.php
• extern/open.php - endpoint to track mail opens via image URL
• extern/rest.php - API endpoint → civicrm/api/v3, civicrm/api/v4 etc
• extern/soap.php - SOAP interface, still used by CiviSMTP? → civicrm/soap
• extern/url.php - Mailing redirect → civicrm/redirect ?
• packages/kcfinder/integration/civicrm.php - KCFinder integration for CiviCRM - IDK

IPN handlers should be tackled as those core payment processors are moved to extensions.