MINOR: h2/trace: indicate 'F' or 'B' to locate the side of an h2c in traces

It was difficult in traces showing h2-to-h2 communications to figure the
connection side solely based on the pointer. With this patch we prepend
'F' or 'B' before the state to make this more explicit:

[06|h2|4|mux_h2.c:5487] h2_rcv_buf(): entering : h2c=0x7f6acc026440(F,FRH) h2s=0x7f6acc021720(1,CLO)
[06|h2|4|mux_h2.c:5547] h2_rcv_buf(): leaving : h2c=0x7f6acc026440(F,FRH) h2s=0x7f6acc021720(1,CLO)
[06|h2|4|mux_h2.c:4040] h2_shutw(): entering : h2c=0x7f6acc026440(F,FRH) h2s=0x7f6acc021720(1,CLO)
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 9ea56c2..c1d18b7 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -503,11 +503,11 @@
 
 	if (src->verbosity > H2_VERB_CLEAN) {
 		if (!h2s || h2c->st0 < H2_CS_FRAME_H)
-			chunk_appendf(&trace_buf, " : h2c=%p(%s)", h2c, h2c_st_to_str(h2c->st0));
+			chunk_appendf(&trace_buf, " : h2c=%p(%c,%s)", h2c, conn_is_back(conn) ? 'B' : 'F', h2c_st_to_str(h2c->st0));
 		else if (h2s->id <= 0)
-			chunk_appendf(&trace_buf, " : h2c=%p(%s) dsi=%d h2s=%p(%d,%s)", h2c, h2c_st_to_str(h2c->st0), h2c->dsi, h2s, h2s->id, h2s_st_to_str(h2s->st));
+			chunk_appendf(&trace_buf, " : h2c=%p(%c,%s) dsi=%d h2s=%p(%d,%s)", h2c, conn_is_back(conn) ? 'B' : 'F', h2c_st_to_str(h2c->st0), h2c->dsi, h2s, h2s->id, h2s_st_to_str(h2s->st));
 		else
-			chunk_appendf(&trace_buf, " : h2c=%p(%s) h2s=%p(%d,%s)", h2c, h2c_st_to_str(h2c->st0), h2s, h2s->id, h2s_st_to_str(h2s->st));
+			chunk_appendf(&trace_buf, " : h2c=%p(%c,%s) h2s=%p(%d,%s)", h2c, conn_is_back(conn) ? 'B' : 'F', h2c_st_to_str(h2c->st0), h2s, h2s->id, h2s_st_to_str(h2s->st));
 	}
 
 	/* Let's dump decoded requests and responses right after parsing. They