winmain.c, bug fix, #315 ngspice under windows 10 shows exception on quit

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.

src/winmain.c

@@ -97,6 +97,8 @@ extern bool ft_ngdebug; /* some additional debug info printed */
 extern bool ft_batchmode;
 extern FILE *flogp;     /* definition see xmain.c, stdout redirected to file */
 
+extern void cp_doquit(void);
+
 #include "winmain.h"
 
 /* --------------------------<history management>------------------------------ */
@@ -506,11 +508,8 @@ MainWindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
         goto DEFAULT_AFTER;
 
     case WM_CLOSE:
-        /* Put Spice commmand "Quit" to end the program into the text buffer */
-        PostSpiceCommand("quit");
-
-        /* If simulation is running, set a breakpoint */
-        raise (SIGINT);
+        cp_doquit();
+        /* continue if the user declined the 'quit' command */
         return 0;
 
     case WM_SIZE: