Register a default value for the timecorrection preference

Register a default value for the timecorrection preference

Previously it had no default value, which means that certain
things did not consider it a real option (e.g. userOptions.php).
Additionally, this was causing preferences to get confused into
thinking that users who did not set a timezone have really selected
offset 00:00, so saving the page could change the user's timezone
to UTC even if they did not touch the setting.

Change-Id: I875c5ab6472324f9bdfdaa784441fbc381087707