Make matching rules theoretically work (add parser).

	* bus/bus.c (bus_context_check_security_policy): fix up to handle
	the case where destination is explicitly specified as bus driver
	and someone else is eavesdropping.

	* bus/policy.c (bus_client_policy_check_can_receive): fix up
	definition of eavesdropping and assertion

	* tools/dbus-send.c (main): use dbus_message_type_from_string

	* bus/signals.c (bus_match_rule_parse): implement

	* dbus/dbus-message.c (dbus_message_type_from_string): new

	* dbus/dbus-errors.h (DBUS_ERROR_MATCH_RULE_INVALID): add