CLEANUP: mux-h2/trace: reformat the "received" messages for better alignment

user-level traces are more readable when visually aligned. This is easily
done by writing "rcvd" instead of "received" to align with "sent" :

  $ socat - /tmp/sock1 <<< "show events buf0"
  [00|h2|0|mux_h2.c:2465] rcvd H2 request  : [1] H2 REQ: GET /?s=10k HTTP/2.0
  [00|h2|0|mux_h2.c:4563] sent H2 response : [1] H2 RES: HTTP/1.1 200
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 7b4f085..42a1e62 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2462,7 +2462,7 @@
 	if (h2s->id > h2c->max_id)
 		h2c->max_id = h2s->id;
 
-	TRACE_USER("received H2 request", H2_EV_RX_FRAME|H2_EV_RX_HDR|H2_EV_STRM_NEW, h2c->conn,, &rxbuf);
+	TRACE_USER("rcvd H2 request ", H2_EV_RX_FRAME|H2_EV_RX_HDR|H2_EV_STRM_NEW, h2c->conn,, &rxbuf);
 	return h2s;
 
  conn_err:
@@ -2539,7 +2539,7 @@
 			h2s_close(h2s);
 	}
 
-	TRACE_USER("received H2 response", H2_EV_RX_FRAME|H2_EV_RX_HDR, h2c->conn,, &h2s->rxbuf);
+	TRACE_USER("rcvd H2 response", H2_EV_RX_FRAME|H2_EV_RX_HDR, h2c->conn,, &h2s->rxbuf);
 	TRACE_LEAVE(H2_EV_RX_FRAME|H2_EV_RX_HDR, h2c->conn, h2s);
 	return h2s;
  fail:
@@ -2899,7 +2899,7 @@
 				b_del(&h2c->dbuf, 1);
 			}
 			h2_skip_frame_hdr(&h2c->dbuf);
-			TRACE_STATE("received H2 frame header", H2_EV_RX_FRAME|H2_EV_RX_FHDR, h2c->conn);
+			TRACE_STATE("rcvd H2 frame header", H2_EV_RX_FRAME|H2_EV_RX_FHDR, h2c->conn);
 
 		new_frame:
 			h2c->dfl = hdr.len;