nifm: treat a joined room as the LAN for local network games

LAN Play through public lobby rooms failed in two ways.

With airplane mode on (the default), IRequest::GetResult rejected the
game's network request. Mario Kart World then opened the net-connect
applet and returned to the main menu instead of the LAN browser. A
joined room is the emulated network: its sockets only reach room
members. Count it as an available network for GetResult and
IsAnyInternetRequestAccepted.

With airplane mode off, players could open the browser but never saw
each other. Room fake IPs are spread over 192.168.0.0/16, and some room
servers give members different /24s. NIFM reported the host adapter's
/24 mask with the fake IP, so the games treated each other as off-subnet.
Report 255.255.0.0 in a room instead.

Assisted-by: Claude Opus 5.5 (investigation and implementation)

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