Add http_argument_multiple_lookup() and http_argument_multiple_free().

Prototypes:
	int	http_argument_multiple_lookup(struct http_req *req,
		    struct http_arg *args);
	void	http_argument_multiple_free(struct http_arg *args);

These functions can be used to lookup arguments in a single call.

args points to an array of struct http_arg elements. Each of them
have the argument name set and its value set to NULL.

The array must have its last element name field set to NULL.

Upon return http_argument_multiple_lookup() gives the caller the
number of arguments that were successfully found. It makes their values
available under the value field in the struct http_arg array passed.

Example:
	int			v;
	struct http_args	args[4];

	memset(args, 0, sizeof(args));
	args[0].name = "email";
	args[1].name = "password1";
	args[2].name = "password2";
	args[3].name = NULL;

	v = http_argument_multiple_lookup(req, args);
	if (v != 3) {
		kore_debug("argument %s was not present", args[v].name);
	} else {
		for (v = 0; args[v].name != NULL; v++)
			kore_debug("%s -> %s", args[v].name, args[v].value);
	}

	http_argument_multiple_free(args);