Recent fixes for URL guessing and building addressed bugs in the code that were preventing the 'baseurlpath' from being used properly. However, they introduced a new issue, as the code was assuming the current URL would always point to a SimpleSAMLphp script. This is not always true, of course, as any script can invoke our API and end up trying to get its own URL (for example, when calling requireAuth()).

In order to fix this, we monitor mismatches between SimpleSAMLphp's installation path and the absolute, real path to the current script. When there's a mismatch, it means we are running a third-party script outside SimpleSAMLphp, and therefore we should NOT enforce 'baseurlpath'. This introduces an additional issue, as applications behind a reverse proxy may cause trouble to guess the right URL (we will use the URL as seen by SimpleSAMLphp in the server, which is not necessarily the same as the user sees with a reverse proxy in between). For the moment, we'll leave the responsibility to sort that issue out to implementors. It might be a good idea to add a page to the wiki explaining how to do this.

This resolves #418.