* dbus/dbus-message-builder.c (_dbus_message_data_load): append
	random signature when using REQUIRED_FIELDS (this hack won't work
	in the long term)

	* dbus/dbus-message.c: change the signature to be a header field,
	instead of message->signature special-case string. Incremental
	step forward. Then we can fix up code to send the signature in the
	message, then fix up code to validate said signature, then fix up
	code to not put the typecodes inline, etc.
	(load_one_message): don't make up the signature after the fact
	(decode_header_data): require signature field for the known
	message types

	* dbus/dbus-marshal.c (_dbus_marshal_string_len): new

	* dbus/dbus-protocol.h: add DBUS_HEADER_FIELD_SIGNATURE