snc: better error messages if pv functions called with multi-PV arrays

We give the hint to use option +p only if the function actually supports
multi-PV arrays. We know this by examining the add_length column in the
builtin function table (functions that support them must add the array
length as implicit argument to the call).

src/snc/gen_ss_code.c

@@ -670,15 +670,31 @@ static void gen_pv_func(
 	if (ap->type == E_VAR)
 	{
 		vp = ap->extra.e_var;
-		if (vp->assign == M_MULTI && !global_options.newpv)
+		assert(vp);
+		if (vp->assign == M_MULTI)
 		{
-			error_at_expr(ap,
-			  "1st argument '%s' to function '%s' must not be a multi-PV array\n",
-			  vp->name, func_name);
-			report_at_expr(ap, "Perhaps you meant %s[0]?\n", vp->name);
-			report_at_expr(ap, "Use option +p to allow this but then "
-			  "pv functions operate on all contained PVs\n");
-			return;
+			if (!add_length)
+			{
+				error_at_expr(ap,
+					"passing multi-PV array '%s' to function '%s' is not "
+					"(yet?) allowed\n",
+					vp->name, func_name);
+				return;
+			}
+			if (!global_options.newpv)
+			{
+				error_at_expr(ap,
+					"passing multi-PV array '%s' to function '%s' is not "
+					"allowed in compatibility mode (option -p)\n",
+					vp->name, func_name);
+				report_at_expr(ap, "Perhaps you meant to pass '%s[0]'?\n", vp->name);
+				if (add_length)
+				{
+					report_at_expr(ap, "Use option +p to allow this but then "
+						"pv functions operate on all contained PVs\n");
+				}
+				return;
+			}
 		}
 	}
 	else if (ap->type == E_SUBSCR)