[FifteenPen]

Opt-in online mode (DM_ONLINE=1) for local stand-in servers. It's off by default, and offline behaviour is unchanged.

• The game's sockets are host sockets. Horizon's errno numbers and O_NONBLOCK are Linux's, not the Mac's (as Online play through Nextendo for NEX and NPLN games: gRPC sockets, MIT sign-in, signed game patches, service profile #2589 found). Poll with a zero timeout always polls once.
• nn::ssl runs over the host's TLS (ssl_online_apple.m), trusting DM_ONLINE_CA.
• nifm reports a network, the account has a network service account (DM_ONLINE_TOKEN), and there is a network clock.
• BCAT is served from a folder (DM_ONLINE_BCAT).
• The game's names resolve to 127.0.0.1 (DM_ONLINE_HOSTS), and port 443 goes to DM_ONLINE_SERVER. DM_ONLINE_PUBLIC=1 lets other names through.
• A new profile key, core_mask, sets GetInfo CoreMask for Switch 2 games whose NPDM allows cores 0-5. The default 0x7 is unchanged.

It was tested with The Duskbloods network test against playDB-CNT's stand-in servers: login, regulation download and the hub all work.