add a variable 'nosavecurrents'.

If set by 'set nosavecurrents' and followed by 'reset',
the setting of internal current vectors is suppressed.
This is useful in ac simulation which does not support
'options savecurrents' and you have a mix of several
simulations in a script.

src/frontend/inp.c

@@ -1984,6 +1984,9 @@ inp_savecurrents(struct card *deck, struct card *options, wordlist *wl, wordlist
     if (!options)
         return wl;
 
+    if (cp_getvar("nosavecurrents", CP_BOOL, NULL, 0))
+        return wl;
+
     /* search for 'save' command in the .control section */
     for (p = controls; p; p = p->wl_next)
         if(prefix("save", p->wl_word))