variable.c, introduce cp_remvar_all() to free all variables

src/frontend/variable.c

@@ -1031,3 +1031,10 @@ var_set_vlist(struct variable *v, struct variable *value)
   v->va_type = CP_LIST;
   v->va_vlist = value;
 }
+
+void
+cp_remvar_all(void)
+{
+    free_struct_variable(variables);
+    variables = NULL;
+}

src/include/ngspice/cpextern.h

@@ -167,6 +167,7 @@ extern void cp_vset(char *varname, enum cp_types type, void *value);
 extern struct variable *cp_setparse(wordlist *wl);
 extern wordlist *vareval(char *string);
 extern char *span_var_expr(char *t);
+extern void cp_remvar_all(void);
 
 /* var2.c */
 extern void cp_vprint(void);