Read IP address from exists tun+ interface

Some routers have an additional interface for tunnel configuration TUN.
Add the ability to read IP addresses from an existing TUN adapter.

sing-box config

  "inbounds": [
    {
      "type": "tun",
      "interface_name": "opkgtun0",
      "domain_strategy": "ipv4_only",
      "auto_route": false,
      "strict_route": false,
      "sniff": true
   }
  ]

ifconfig opkgtun0

opkgtun0  Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
          inet addr:172.16.0.2  P-t-P:172.16.0.2  Mask:255.255.255.255
          inet6 addr: fadd:4700:110:::2/128 Scope:Global
          POINTOPOINT NOARP MULTICAST  MTU:65535  Metric:1
          RX packets:219 errors:0 dropped:0 overruns:0 frame:0
          TX packets:254 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:150

Now

 ~  # sing-box run --config /tmp/1.json --directory /tmp
INFO[0000] network: updated default interface eth3, index 7
TRACE[0000] inbound/tun[0]: creating stack
FATAL[0000] start service: start inbound/tun[0]: missing interface address

Start at the TUN inbound startup path that produces the “missing interface address” error, then trace how addresses are discovered for an existing interface. Reproduce with the provided sing-box configuration and opkgtun0 setup; done means the existing TUN adapter’s IPv4 and IPv6 addresses are read without the startup failure.