Fix timezone initialization in core.php

Prior to this, core.php would trigger a PHP warning on PHP >= 5.3 when
date_default_timezone_get() is not able to determine the timezone and
defaults to UTC (e.g. when php.ini date.timezone is not set).

The timezone is determined in the following order

1. user preferences
2. MantisBT configuration ($g_default_timezone)
3. php.ini setting (date.timezone)
4. UTC (set by date_default_timezone_get())

Fixes 0017747