Added support for strings_inc.php, if this file is found in main directory it gets
included after strings_xxxx.php to allow overriding of strings without modifying
the language files. $g_active_language can be used to check the active
language.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1134 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9