hdf5_ep

[svn-r17976] Bug fix for 1584.  H5Tdetect_class said a VL string is a string type.  But when it's in a 
compound type, it says it's a VL type.  We want to tell user a VL string is a string.  But 
internally we treat it as a VL type.  I added a flag as a parameter of H5T_detect_class.  It
tells whether the caller is the public function H5Tdetect_class.  I also added a detection for
VL string in the private function for the compound case (or array or nested VL type).

Tested with h5committest.