MINOR: h2: clarify the fact that the send functions are unsigned

There's no more error return combined with the send output, though
the comments were misleading. Let's fix this as well as the functions'
prototypes. h2_snd_buf()'s return value wasn't changed yet since it
has to match the ->snd_buf prototype.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index beb7dca..fb509b9 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2939,11 +2939,11 @@
 }
 
 /* Try to send a HEADERS frame matching HTTP/1 response present in buffer <buf>
- * for the H2 stream <h2s>. Returns 0 if not possible yet, <0 on error (one of
- * the H2_ERR* or h2_status codes), >0 on success in which case it corresponds
- * to the number of buffer bytes consumed.
+ * for the H2 stream <h2s>. Returns the number of bytes sent. The caller must
+ * check the stream's status to detect any error which might have happened
+ * subsequently to a successful send.
  */
-static int h2s_frt_make_resp_headers(struct h2s *h2s, struct buffer *buf)
+static size_t h2s_frt_make_resp_headers(struct h2s *h2s, struct buffer *buf)
 {
 	struct http_hdr list[MAX_HTTP_HDR];
 	struct h2c *h2c = h2s->h2c;
@@ -3112,17 +3112,17 @@
 }
 
 /* Try to send a DATA frame matching HTTP/1 response present in the response
- * buffer <buf>, for stream <h2s>. Returns 0 if not possible yet, <0 on error
- * (one of the H2_ERR* or h2_status codes), >0 on success in which case it
- * corresponds to the number of buffer bytes consumed.
+ * buffer <buf>, for stream <h2s>. Returns the number of bytes sent. The caller
+ * must check the stream's status to detect any error which might have happened
+ * subsequently to a successful send.
  */
-static int h2s_frt_make_resp_data(struct h2s *h2s, struct buffer *buf)
+static size_t h2s_frt_make_resp_data(struct h2s *h2s, struct buffer *buf)
 {
 	struct h2c *h2c = h2s->h2c;
 	struct h1m *h1m = &h2s->res;
 	struct chunk outbuf;
 	int ret = 0;
-	int total = 0;
+	size_t total = 0;
 	int es_now = 0;
 	int size = 0;
 	char *blk1, *blk2;
@@ -3357,7 +3357,7 @@
 	}
 
  end:
-	trace("[%d] sent simple H2 DATA response (sid=%d) = %d bytes out (%d in, st=%s, ep=%u, es=%s, h2cws=%d h2sws=%d) buf->o=%u", h2c->st0, h2s->id, size+9, total, h1_msg_state_str(h1m->state), h1m->err_pos, h1_msg_state_str(h1m->err_state), h2c->mws, h2s->mws, (unsigned int)buf->o);
+	trace("[%d] sent simple H2 DATA response (sid=%d) = %d bytes out (%u in, st=%s, ep=%u, es=%s, h2cws=%d h2sws=%d) buf->o=%u", h2c->st0, h2s->id, size+9, (unsigned int)total, h1_msg_state_str(h1m->state), h1m->err_pos, h1_msg_state_str(h1m->err_state), h2c->mws, h2s->mws, (unsigned int)buf->o);
 	return total;
 }
 
@@ -3365,7 +3365,7 @@
 static int h2_snd_buf(struct conn_stream *cs, struct buffer *buf, int flags)
 {
 	struct h2s *h2s = cs->ctx;
-	int total = 0;
+	size_t total = 0;
 
 	if (!(h2s->flags & H2_SF_OUTGOING_DATA) && buf->o)
 		h2s->flags |= H2_SF_OUTGOING_DATA;