MDL-15476

Can now override the public key lifetime by specificying value
for $CFG->mnetkeylifetime in config.php

Merged from STABLE_19

config-dist.php

@@ -132,6 +132,13 @@ $CFG->admin = 'admin';
 // These are additional tweaks for which no GUI exists in Moodle yet.
 //
 //
+// Change the key pair lifetime for Moodle Networking
+// The default is 28 days. You would only want to change this if the key
+// was not getting regenerated for any reason. You would probably want
+// make it much longer. Note that you'll need to delete and manually update
+// any existing key.
+//      $CFG->mnetkeylifetime = 28;
+//
 // Prevent scheduled backups from operating (and hide the GUI for them)
 // Useful for webhost operators who have alternate methods of backups
 //      $CFG->disablescheduledbackups = true;