Fix Calc graph output on MS-Windows

Fix Calc graph output on MS-Windows

The previous code relied on "pgnuplot" executable, which is
no longer provided with Gnuplot 5.x.
* lisp/calc/calc.el (calc-gnuplot-name): Set to "pgnuplot" on
MS-Windows only if such an executable exists.
* lisp/calc/calc-graph.el (calc-graph-w32-p): New defsubst.
(calc-graph-plot, calc-graph-command, calc-gnuplot-command)
(calc-graph-init): Call calc-graph-w32-p wherever we need to
do something special for invoking gnuplot on MS-Windows,
instead of comparing against calc-gnuplot-name.
(calc-graph-plot): Set the terminal to "qt" on MS-Windows when
pgnuplot.exe is not available.
(calc-graph-kill): Delete the temporary files only after
killing the gnuplot process, otherwise the deletion might fail
on MS-Windows because the files are still in use.