[airplay] Add support for AirPlay 2

Includes
- Implementation in src/outputs/airplays2, type OUTPUT_TYPE_AIRPLAY
- Homekit pairing, both normal (with PIN) and transient
- New session startup sequence, incl GET /info, SETPEERS and 2 x SETUP
- No more OPTIONS and ANNOUNCE
- Use POST /feedback for keepalive instead of SET_PARAMETERS
- Sequence dispatching instead of callback chains
- Continue despite "Bad request" to SET_PARAMETER (volume)
- Opening of event connection to receiver (reverse rtsp connection)

Still to be done
- Password authentication
- Handling of events

src/evrtsp/evrtsp.h

@@ -45,8 +45,9 @@ extern "C" {
 
 /* Response codes */
 #define RTSP_OK			200
-#define RTSP_UNAUTHORIZED       401
-#define RTSP_FORBIDDEN          403
+#define RTSP_UNAUTHORIZED              401
+#define RTSP_FORBIDDEN                 403
+#define RTSP_CONNECTION_AUTH_REQUIRED  470
 
 struct evrtsp_connection;
 
@@ -135,6 +136,10 @@ void evrtsp_connection_free(struct evrtsp_connection *evcon);
 void evrtsp_connection_set_closecb(struct evrtsp_connection *evcon,
     void (*)(struct evrtsp_connection *, void *), void *);
 
+/** Set a callback for encryption/decryption. */
+void evrtsp_connection_set_ciphercb(struct evrtsp_connection *evcon,
+    void (*)(struct evbuffer *, void *, int encrypt), void *);
+
 /**
  * Associates an event base with the connection - can only be called
  * on a freshly created connection object that has not been used yet.

src/evrtsp/rtsp-internal.h

@@ -85,6 +85,9 @@ struct evrtsp_connection {
 	void (*closecb)(struct evrtsp_connection *, void *);
 	void *closecb_arg;
 
+	void (*ciphercb)(struct evbuffer *evbuf, void *, int encrypt);
+	void *ciphercb_arg;
+
 	struct event_base *base;
 };
 

src/evrtsp/rtsp.c

@@ -620,6 +620,9 @@ evrtsp_read(int fd, short what, void *arg)
 		return;
 	}
 
+	if (evcon->ciphercb)
+		evcon->ciphercb(evcon->input_buffer, evcon->ciphercb_arg, 0);
+
 	switch (evcon->state) {
 	case EVCON_READING_FIRSTLINE:
 		evrtsp_read_firstline(evcon, req);
@@ -725,6 +728,10 @@ evrtsp_request_dispatch(struct evrtsp_connection* evcon)
 	/* Create the header from the store arguments */
 	evrtsp_make_header(evcon, req);
 
+	/* forked-daapd customisation for encryption */
+	if (evcon->ciphercb)
+		evcon->ciphercb(evcon->output_buffer, evcon->ciphercb_arg, 1);
+
 	evrtsp_write_buffer(evcon, evrtsp_write_connectioncb, NULL);
 }
 
@@ -1312,6 +1319,14 @@ evrtsp_connection_set_closecb(struct evrtsp_connection *evcon,
 	evcon->closecb_arg = cbarg;
 }
 
+void
+evrtsp_connection_set_ciphercb(struct evrtsp_connection *evcon,
+    void (*cb)(struct evbuffer *, void *, int encrypt), void *cbarg)
+{
+	evcon->ciphercb = cb;
+	evcon->ciphercb_arg = cbarg;
+}
+
 void
 evrtsp_connection_get_local_address(struct evrtsp_connection *evcon,
     char **address, u_short *port, int *family)