Explicitly globalize $table_prefix in wp-settings.php in case WordPress isn't...

Explicitly globalize $table_prefix in wp-settings.php in case WordPress isn't loaded in global scope. Use $_SERVER['PHP_SELF'], not $PHP_SELF. We need to keep $PHP_SELF for backwards compatibility reasons (many, many plugins rely on it). props mrtorrent, fixes #17749.

git-svn-id: https://core.svn.wordpress.org/trunk@18993 1a063a9b-81f0-0310-95a4-ce76da25c4cd