network: gate nwc_service_t to server/nettool only

[janry]

There is a special packet that crashes the server. This fix closes that.

    The wire-symmetric rdwr previously let any TCP peer drive
    socket_list_t::rdwr / address_list_t::rdwr through an unbounded
    allocation loop with a wire-supplied count, before the admin-auth
    gate in execute() fired — one packet, server gone. The honest
    statement is narrower than per-flag asymmetry: nwc_service_t is the
    admin-tool <-> server channel, and an in-game multiplayer client
    never legitimately exchanges one. Early-return on that side closes
    the pre-auth server crash and the symmetric malicious-server push
    at a joined client in one shot. SRVC_GET_BLACK_LIST patched together
    with SRVC_GET_CLIENT_LIST.

[prissi]

Thank you, your work will certainly reduce the spurious server crashes. I was happy enough to get the network working, and happy for the extended interface from Dwachs. Since then, not many people looked at it.
Incorporated in r11961.