Fix some annoying DBusString API and fix all affected code.

	* dbus/dbus-string.c (_dbus_string_init): get rid of annoying
	max_length argument
	(_dbus_string_get_data): change to return string instead of using
	an out param
	(_dbus_string_get_const_data): ditto
	(_dbus_string_get_data_len): ditto
	(_dbus_string_get_const_data_len): ditto