fix: disable strongswan autostart in postinst

1. Add `systemctl disable strongswan` to dcc-network-plugin.postinst,
   alongside the existing ipsec/xl2tpd/openvpn disables.
2. strongSwan 6.x ships only a native systemd unit (strongswan.service),
   no LSB init script, so update-rc.d cannot handle it; use systemctl
   disable instead.
3. The `ipsec` entry maps to /etc/init.d/ipsec from strongswan-starter, the
   legacy SysV stack; the `strongswan` metapackage instead installs the
   systemd stack (charon-systemd + strongswan-swanctl), so that entry is a
   no-op here. strongswan comes in via the network-manager-strongswan
   dependency, and its strongswan.service declares
   WantedBy=multi-user.target, so it autostarts once installed.

Log: unreviewed strongswan.service no longer autostarts (security baseline)

Influence:
1. Fresh install: `systemctl list-unit-files | grep strongswan` shows
   disabled; strongswan.service must not autostart at boot.
2. Upgrade existing install: postinst re-run emits no error and
   strongswan.service ends up disabled.
3. IPsec VPN via NetworkManager still works (backend started on demand).

PMS: BUG-379501

fix: postinst 中禁用 strongswan 自启

1. 在 dcc-network-plugin.postinst 中新增 `systemctl disable strongswan`，
   与已有的 ipsec/xl2tpd/openvpn 禁用逻辑并列。
2. strongSwan 6.x 仅提供原生 systemd 单元（strongswan.service），无 LSB
   init 脚本，update-rc.d 无法处理，需改用 systemctl disable。
3. 上面的 `ipsec` 对应 strongswan-starter 提供的 /etc/init.d/ipsec，属传统
   SysV 栈；而 `strongswan` 元包实际安装的是 systemd 栈（charon-systemd +
   strongswan-swanctl），故该条在此为空操作。strongswan 经
   network-manager-strongswan 依赖引入，其 strongswan.service 声明
   WantedBy=multi-user.target，安装即开机自启。

Log: 系统自启服务不再包含未经评审的 strongswan.service（安全基线）

Influence:
1. 全新安装：`systemctl list-unit-files | grep strongswan` 应为 disabled，
   开机不再自启 strongswan.service。
2. 升级安装：重跑 postinst 无报错，strongswan.service 被禁用。
3. 通过 NetworkManager 使用 IPsec VPN 仍正常（后端按需启动）。

PMS: BUG-379501

Author: caixr23

debian/dcc-network-plugin.postinst

@@ -6,6 +6,7 @@ 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
+    systemctl disable strongswan || true
     mkdir -p /var/cache/deepin
     touch /var/cache/deepin/mark-setup-network-services
 fi