MINOR: h2/htx: Set the flag HTX_SL_F_BODYLESS for messages without body

This information is usefull to know if a body is expected or not, regardless the
presence or not of the header "Content-Length" and its value. Once the ES flag
is set on the header frame or when the content length is 0, we can safely add
the flag HTX_SL_F_BODYLESS on the HTX start-line.

Among other things, it will help the mux-h1 to know if it should add TE header
or not. It will also help the HTTP compression filter.

This patch must be backported to 1.9 because a bug fix depends on it.
diff --git a/src/h2.c b/src/h2.c
index 6a1debf..d7031c7 100644
--- a/src/h2.c
+++ b/src/h2.c
@@ -684,6 +684,9 @@
 			goto fail;
 	}
 
+	if (!(*msgf & H2_MSGF_BODY) || ((*msgf & H2_MSGF_BODY_CL) && *body_len == 0))
+		sl_flags |= HTX_SL_F_BODYLESS;
+
 	/* update the start line with last detected header info */
 	sl->flags |= sl_flags;
 
@@ -890,6 +893,9 @@
 			goto fail;
 	}
 
+	if (!(*msgf & H2_MSGF_BODY) || ((*msgf & H2_MSGF_BODY_CL) && *body_len == 0))
+		sl_flags |= HTX_SL_F_BODYLESS;
+
 	/* update the start line with last detected header info */
 	sl->flags |= sl_flags;