security MDL-18006 MDL-18807 MDL-20853 Several improvements to increase security

*  Installation
      o Adds a random $CFG->passwordsaltmain to config.php on install
      o Changes passwordpolicy default to on
* Upgrade
      o If passwordpolicy is off it switches it on
      o Checks if passwordsaltmain has been set and if not recommends to the user and all admins to add it, both on the upgrade output and via messaging.
      o Forces all admin users to change their password at next login.
* General
      o Extends the security overview report to check for passwordmainsalt and recommend if it not set (or weak)

admin/index.php

@@ -635,6 +635,10 @@
         
     }
 
+    if (empty($CFG->passwordsaltmain)) {
+        print_box(get_string('upgrade197notice', 'admin')."\n".get_string('upgrade197salt', 'admin'));
+    }
+
     if (defined('WARN_DISPLAY_ERRORS_ENABLED')) {
         print_box(get_string('displayerrorswarning', 'admin'), 'generalbox adminwarning');
     }