MINOR: connection: add a flags argument to rcv_buf()

The mux and transport rcv_buf() now takes a "flags" argument, just like
the snd_buf() one or like the equivalent syscall lower part. The upper
layers will use this to pass some information such as indicating whether
the buffer is free from outgoing data or if the lower layer may allocate
the buffer itself.
diff --git a/src/raw_sock.c b/src/raw_sock.c
index daf3d07..477862e 100644
--- a/src/raw_sock.c
+++ b/src/raw_sock.c
@@ -250,7 +250,7 @@
  * errno is cleared before starting so that the caller knows that if it spots an
  * error without errno, it's pending and can be retrieved via getsockopt(SO_ERROR).
  */
-static size_t raw_sock_to_buf(struct connection *conn, struct buffer *buf, size_t count)
+static size_t raw_sock_to_buf(struct connection *conn, struct buffer *buf, size_t count, int flags)
 {
 	ssize_t ret;
 	size_t try, done = 0;