COMP: Added istringstream::clear() method to disambiguate the call from using string::clear or istrstream::clear.

ENH: Added KWSYSPE_DEBUG macro to print debugging trace information.  Added TODO comment explaining why process execution can still hang when a grandchild keeps the output pipes open.

COMP: Fix warning about binding reference-to-non-const to an rvalue on VS6.  It does not seem to be doing the proper auto_ptr_ref conversions.  Instead use the const_cast work-around on this platform.