More work on wire protocol

Make separate payload for the header.
Make release-fd payloads
capture_buffer -> peek_buffer to avoid a copy
remove release-buffer, we really need to release each fd in the buffer
separately.
provide_buffer -> send_buffer so that we can also use this to send the
release-fd messages.
in pinossrc, send back release-fd messages when the fd is no longer in
use.

src/client/private.h

@@ -57,7 +57,6 @@ GDBusProxy *           pinos_subscribe_get_proxy_finish   (PinosSubscribe *subsc
 
 
 typedef struct {
-  PinosBufferHeader header;
   guint32 version;
   guint32 length;
 } PinosStackHeader;