fix for 0003384: $g_path setting in config_inc.php will NOT override default

• also replaced most instances of $g_absolute_path except for $g_core_path
• thanks to ryandesign for pointing this out

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@3474 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9