Logging: allow logging with empty varargs.

Currently, the log macro requires at least one argument after the format string,
because of the way the preprocessor handles varargs macros.  We can hide some of
that irritation by pushing the extra arguments into a varargs function.

test/unit/log.c

@@ -170,13 +170,25 @@ TEST_BEGIN(test_logs_if_no_init) {
 }
 TEST_END
 
+/*
+ * This really just checks to make sure that this usage compiles; we don't have
+ * any test code to run.
+ */
+TEST_BEGIN(test_log_only_format_string) {
+	if (false) {
+		static log_var_t l = LOG_VAR_INIT("str");
+		log(l, "No arguments follow this format string.");
+	}
+}
+TEST_END
+
 int
 main(void) {
-
 	return test(
 	    test_log_disabled,
 	    test_log_enabled_direct,
 	    test_log_enabled_indirect,
 	    test_log_enabled_global,
-	    test_logs_if_no_init);
+	    test_logs_if_no_init,
+	    test_log_only_format_string);
 }