Check if shutdown_request can be implemented more gracefully by backends

[zmughal]

An example of this causing issues is when running PDL::Graphics::Gnuplot which has a line:

https://github.com/PDLPorters/PDL-Graphics-Gnuplot/blob/2.032/lib/PDL/Graphics/Gnuplot.pm#L2102

which dies on various signals. This creates extra messages even in non-interactive settings such as using jupyter nbconvert. It can also be seen with jupyter notebook when terminating kernels manually or shutting down the server.

Docs: https://ipython.org/ipython-doc/dev/development/messaging.html#kernel-shutdown.

[mohawk2]

This could be handled at least in P:G:Gnuplot by stopping that signal-handling if an environment variable is set, maybe PDL_GRAPHICS_GNUPLOT_NO_SIGHANDLE, which your plugin could set?