PHP-1226: Add support for the new listIndexes command in 2.7.5

Some of this is also in #741 , which should be merged first.
Where is `retval` initialized to an empty array (in advance of `RETURN_ZVAL(retval, 0, 1);` a few lines down? There should probably be a test for this behavior, too. I don't believe any of the included phpt files tested for it.