Stop using SimpleSAML_Configuration::getBaseURL().

Use the recently added SimpleSAML_Configuration::getBasePath() instead. It guarantees the path prepended with a slash, so no need to do that every time when calling the method. As a side effect, we get rid of buggy invocations (calling getBaseUrl() instead of getBaseURL()), and also of old-style convention for the 'baseurlpath' configuration option, allowing a star at the beginning.