Remove base64 encoding, replace with hex encoding. Original patch
	from trow@ximian.com, added error handling.

	* dbus/dbus-string.c (_dbus_string_base64_encode)
	(_dbus_string_base64_decode): Remove.
	(_dbus_string_hex_decode): Add end_return argument so we can
	distinguish between OOM and invalid hex encoding.
	(_dbus_string_test): Remove base64 tests and add test case for
	invalid hex.

	* dbus/dbus-keyring.c, dbus/dbus-auth-script.c, dbus/dbus-auth.c:
	Replace base64 with hex.

	* test/data/auth/invalid-hex-encoding.auth-script: New test case
	for invalid hex encoded data in auth protocol.