MDL-8840 Loginas cleanup, some more improvements; merged from MOODLE_18_STABLE

+ merging of format_string fixes

course/enrol.php

@@ -27,6 +27,11 @@
         error("That's an invalid course id");
     }
 
+/// do not use when in course login as
+    if (!empty($USER->realuser) and $USER->loginascontext->contextlevel == CONTEXT_COURSE) {
+        print_error('loginasnoenrol', '', $CFG->wwwroot.'/course/view.php?id='.$USER->loginascontext->instanceid);
+    }
+
     $enrol = enrolment_factory::factory($course->enrol); // do not use if (!$enrol... here, it can not work in PHP4 - see MDL-7529 
 
 /// Refreshing all current role assignments for the current user