session: support half-close connection

Some app(e.g. Envoy) may call shutdown() instead of close() when
draining connection.

Type: improvement

Signed-off-by: liuyacan <liuyacan@corp.netease.com>
Change-Id: I9543b9ca3caa87b10b134fd1fc4019124e41e4d2
diff --git a/src/vcl/ldp.c b/src/vcl/ldp.c
index 64a4e7c..f27f6ba 100644
--- a/src/vcl/ldp.c
+++ b/src/vcl/ldp.c
@@ -2219,6 +2219,8 @@
 
       if (flags == SHUT_RDWR)
 	rv = close (fd);
+      else if (flags == SHUT_WR)
+	rv = vls_shutdown (vlsh);
     }
   else
     {