https://sourceforge.net/p/ngspice/bugs/315/
longjmp() creates segfault under Windows 10 (VS and MINGW)
somewhere when MainWindowProc() is processing the WM_CLOSE message.
Instead of raising SIGINT, directly call cp_doquit().
return 0 if "askquit" is set and the user declines to "quit".
This continues the simulation, if still active.