misccoms.c, com_quit(), don't free memory, unless shared ngspice

Traversing all linked memory structures to free their memory
  can be a somewhat lengthy business, especially in xspice,
  which is not worth the effort when we simply want to exit()
Long delays have been reported in certain cases.

For developers and for the usage of such tools as valgrind,
  we still free all the memory if 'set ngdebug' is given.

src/frontend/misccoms.c

@@ -56,6 +56,11 @@ com_quit(wordlist *wl)
         if (!noask && !confirm_quit())
             return;
 
+#ifndef SHARED_MODULE
+	if (!ft_ngdebug)
+	    exit(exitcode);
+#endif
+
     /* start to clean up the mess */
 
 #ifdef SHARED_MODULE