Add SO_REUSEPORT support for macOS/BSD NetSIO compatibility

On macOS and BSD systems, SO_REUSEPORT is required for multiple processes
to bind to the same UDP port (9997 for NetSIO). Without this flag, only
the first process to bind succeeds, preventing bidirectional NetSIO
communication.

This change:
  - Adds SO_REUSEPORT socket option for macOS/BSD builds
  - Maintains compatibility with existing code (guarded by #ifdef)
  - Uses standard #ifdef SO_REUSEPORT guard for automatic platform detection - no build system changes required. Works on macOS, BSD, and modern Linux.
  - Enables proper NetSIO operation when emulator and FujiNet-PC both need to bind to port 9997

Author: pedgarcia

lib/tcpip/fnUDP.cpp

@@ -99,6 +99,14 @@ bool fnUDP::begin(in_addr_t address, uint16_t port)
         return false;
     }
 
+    /* On macOS/BSD, SO_REUSEPORT is required to allow multiple processes to bind to the same port */
+#ifdef SO_REUSEPORT
+    if (setsockopt(udp_server, SOL_SOCKET, SO_REUSEPORT, &yes, sizeof(yes)) < 0)
+    {
+        Debug_printf("could not set SO_REUSEPORT: %d\r\n", compat_getsockerr());
+    }
+#endif
+
     struct sockaddr_in addr;
     memset(&addr, 0, sizeof(addr));