feat: add support for setting custom udp listen port on the xtcp mode

[zjay1987]

Enhancing xtcp Connectivity with UDP Port Forwarding

When your home router’s NAT traversal (hole punching) doesn’t work well—such as when it operates in Symmetric NAT mode only—you can configure a UDP port forwarding rule on the router as a workaround. For example:

Router Configuration

UDP 9999 → {Your-LAN-IP}:9999

frpc Configuration

[[visitors]]
name = "..."
type = "xtcp"
serverName = "..."
secretKey = "..."
bindAddr = "..."
bindPort = "..."
udpListen = "0.0.0.0:9999"

With this setup, frpc will use UDP port 9999 for communication with the STUN server instead of selecting a random port. Since port 9999 on the router is already configured to forwarded all traffics to {Your-LAN-IP}:9999, the likelihood of establishing a successful xtcp connection increases significantly.