reduce dependencies on driver/rcinput.h

* add new header that only contains neutrino_msg_t and friends, often this
  is all that's needed instead of full rcinput.h
* directly include rcinput.h in some cpp files instead of relying on
  accidental inclusion in some header
* add class forward declarations to avoid dragging in rcinput indirectly
This could use more work to further reduce the impact; maybe separating
the CRCinput::key_* constants from the rcinput class would be good.

src/driver/neutrino_msg_t.h

@@ -0,0 +1,20 @@
+/*
+ * (C) 2018 Stefan Seyfried
+ * SPDX-License-Identifier: GPL-2.0
+ *
+ * define neutrino_msg_t and friends
+ * pulled out of rcinput.h to reduce impact of that header
+ */
+
+#ifndef __neutrino_msg_t_h__
+#define __neutrino_msg_t_h__
+
+typedef unsigned long neutrino_msg_t;
+typedef unsigned long neutrino_msg_data_t;
+
+/* ugly hack to avoid including rcinput.h for key default values */
+#define RC_NOKEY 0xFFFFFFFE
+
+#define NEUTRINO_UDS_NAME "/tmp/neutrino.sock"
+
+#endif