AG disk: use the query path, not the request path, for looking up files
on disk.
Add AG_driver_get_query_string_mi() to look up query string given an
AG_map_info structure.
Added some documentation; cleaned up some memory management in
map_info.cpp
Remove gratuitous ListDir()--only ListDir() if we know it's a directory.
Also, make page_id a mandatory argument to file_resolve(), the /RESOLVE
API, and the ListDir() method.
Updated index
Return -ENOENT if a listing was not found.  Correctly handle all types
of listing return values.  Clean up listing memory management.