fix: migrate VPN backend disable logic from network-manager-integration-plugins

The postinst from network-manager-integration-plugins (0.0.5-0deepin) was
not migrated when its dependency was replaced with direct NM plugin deps
in 581b0c6, leaving ipsec/xl2tpd/openvpn init.d services enabled on
fresh installs.

fix: 迁移 network-manager-integration-plugins 的 VPN 后端禁用逻辑

581b0c6 移除 network-manager-integration-plugins 依赖时未迁移其 postinst
的 VPN 后端禁用逻辑，导致 ipsec/xl2tpd/openvpn 的 init.d 服务保持启用。
本提交将 postinst 原样迁移到 dcc-network-plugin，安装时自动禁用这三个服务。

Log: 迁移 VPN 后端禁用逻辑到 dcc-network-plugin
PMS: BUG-379501
Influence: 安装 dcc-network-plugin 后首次配置会禁用 ipsec/xl2tpd/openvpn
的 init.d 服务，由 NetworkManager 统一管理 VPN 后端，避免重复自启。

Author: caixr23

debian/dcc-network-plugin.postinst

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+#Disable the network manager plugin backend service.
+#NetworkManager will start service automatic.
+if [ ! -e /var/cache/deepin/mark-setup-network-services ];then
+    update-rc.d ipsec disable || true
+    update-rc.d xl2tpd disable || true
+    update-rc.d openvpn disable || true
+    mkdir -p /var/cache/deepin
+    touch /var/cache/deepin/mark-setup-network-services
+fi
+
+#DEBHELPER#
+
+exit 0