xl2tpd: fix invoking xl2tpd-control

Upstream commit 90884c62 ("xl2tpd-control refactoring") introduced in
1.3.16 changed command names

The l2tp protocol handler part was from @danvd in pull request
openwrt/packages#13866

Fixes f07319d6 ("xl2tpd: bump to version 1.3.16")

Ref: https://github.com/openwrt/packages/pull/13866
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

net/xl2tpd/patches/0001-xl2tpd-control-fix-out-of-bound-access.patch

@@ -0,0 +1,26 @@
+From f94ffac18ddfe72b00ec849a34c71d6bc5bcb767 Mon Sep 17 00:00:00 2001
+From: Yousong Zhou <yszhou4tech@gmail.com>
+Date: Thu, 12 Nov 2020 14:31:23 +0800
+Subject: [PATCH] xl2tpd-control: fix out-of-bound access
+
+It can cause segfault when presented with unknown command name
+
+Fixes 90884c62 ("xl2tpd-control refactoring")
+---
+ xl2tpd-control.c | 3 ++-
+ 1 file changed, 2 insertions(+), 1 deletion(-)
+
+diff --git a/xl2tpd-control.c b/xl2tpd-control.c
+index 1e87c93..d1df0f6 100644
+--- a/xl2tpd-control.c
++++ b/xl2tpd-control.c
+@@ -83,7 +83,8 @@ struct command_t commands[] = {
+     {"available",     &command_available,     TUNNEL_NOT_REQUIRED},
+     {"add-lns",       &command_add_lns,       TUNNEL_REQUIRED},
+     {"status-lns",    &command_status_lns,    TUNNEL_REQUIRED},
+-    {"remove-lns",    &command_remove_lns,    TUNNEL_REQUIRED}
++    {"remove-lns",    &command_remove_lns,    TUNNEL_REQUIRED},
++    {},
+ };
+ 
+ void usage()