MEDIUM: mux: make mux->rcv_buf() take a size_t for the count

It also returns a size_t. This is in order to clean the API. Note
that the H2 mux still uses some ints in the functions called from
h2_rcv_buf(), though it's not really a problem given that H2 frames
are smaller. It may deserve a general cleanup later though.
diff --git a/include/types/connection.h b/include/types/connection.h
index 69d7394..18c2eb0 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -297,7 +297,7 @@
 	void (*send)(struct connection *conn);        /* mux-layer send callback */
 	int  (*wake)(struct connection *conn);        /* mux-layer callback to report activity, mandatory */
 	void (*update_poll)(struct conn_stream *cs);  /* commit cs flags to mux/conn */
-	int  (*rcv_buf)(struct conn_stream *cs, struct buffer *buf, int count); /* Called from the upper layer to get data */
+	size_t (*rcv_buf)(struct conn_stream *cs, struct buffer *buf, size_t count); /* Called from the upper layer to get data */
 	size_t (*snd_buf)(struct conn_stream *cs, const struct buffer *buf, size_t count, int flags); /* Called from the upper layer to send data */
 	int  (*rcv_pipe)(struct conn_stream *cs, struct pipe *pipe, unsigned int count); /* recv-to-pipe callback */
 	int  (*snd_pipe)(struct conn_stream *cs, struct pipe *pipe); /* send-to-pipe callback */