Add option to enable SO_REUSEPORT for co-operating with natmap on router.

[DiruSec]

natmap was used for TCP / UDP hole punching under a NAT1 environment.
For communication, user should:

• Listen to a port bind by natmap, or
• Add port forwarding (DNAT) rules to firewall, or
• Use build-in forward method.

After some tries, seems only the first method can co-operate with udp2raw.
No data can be received by udp2raw server using port forwarding provided by firewall, and udp2raw client and server cannot establish connection using natmap's build-in forward feature.

#201 talked about add SO_REUSEPORT for performance purpose. But seems SO_REUSEPORT is still not enabled at current.

natmap's solution for bind same port requires Linux kernel 5.6 or newer, though there's a number of routers using kernel older than 5.6.

I've compiled an udp2raw binary with SO_REUSEPORT enabled and it just works, so any plans for adding it as an option?

[wangyu-]

I've compiled an udp2raw binary with SO_REUSEPORT enabled and it just works, so any plans for adding it as an option?

can you show me which socket did you enable SO_REUSEPORT? (is it only server's listen socket?)

I can add it as an option in next release.