Add opt-in reuseport metadata option for tcp, mtcp, and redirect/tcp
listeners, allowing a GOST listener on a specific IP to coexist with
another process on the same port but different address (e.g., Nginx on
0.0.0.0:443 + GOST on 127.0.0.1:443). With SO_REUSEPORT, the kernel
routes to the most specific address match.
On Unix, sets SO_REUSEPORT via ListenConfig.Control before bind().
On Windows, the option is a no-op (SO_REUSEADDR is the default).
Closesgo-gost/gost#654
ginuerzh
99a0f7323d