MINOR: mux: Add info about the supported side in alpn_mux_list structure

Now, a multiplexer can specify if it can be install on incoming connections
(ALPN_SIDE_FE), on outgoing connections (ALPN_SIDE_BE) or both
(ALPN_SIDE_BOTH). These flags are compatible with proxies' ones.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index f46504c..192578b 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -3603,7 +3603,7 @@
 
 /* ALPN selection : this mux registers ALPN tolen "h2" */
 static struct alpn_mux_list alpn_mux_h2 =
-	{ .token = IST("h2"), .mode = ALPN_MODE_HTTP, .mux = &h2_ops };
+	{ .token = IST("h2"), .mode = ALPN_MODE_HTTP, .side = ALPN_SIDE_FE, .mux = &h2_ops };
 
 /* config keyword parsers */
 static struct cfg_kw_list cfg_kws = {ILH, {