ENH: Added more special unix shell characters that require quoting.  Added escaping of % as %% for shells inside mingw32-make.

BUG: Do not send both SIGSTOP and SIGKILL when killing a process.  The SIGSTOP seems to be able to block the SIGKILL occasionally.  Also the SIGKILL is sufficient since the process table entry will still exist until it is reaped with waitpid.

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.