support $g_dsn for data source name strings to be supplied for adodb.
fix: 0005956: Database system scanner via variable poisoning
minor performance increase in php in db_fetch_array

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@3686 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9