Let envd listen on a Unix socket

[cptspacemanspiff]

I have started working on a new environment provider wrapping SmolVM (I want to have something I can run cross-platform, and that seems to have that abstraction, and on user machines). As part of this I want to have the ability to fully lock-down the VM's network access, so envd only allowing TCP endpoints is an issue. To solve this I add the ability to use a unix domain socket as the endpoint, which a VM runtime can connect to from the other side.

For context here is the current (rough) additional provider: https://github.com/cptspacemanspiff/lightspeed-provider-smolvm

The only thing not directly related to unix domain socket, addition was that instead of crashing envd on 'out of file descriptors' or 'connect' errors, there is a sleep + error log, so that deamon and it's existing session connections won't get killed if there is something goes wrong with a new connection.

From Claude:

A host that reaches the daemon over a local bridge rather than the network, such as a microVM whose socket is relayed over vsock, needs envd to serve a Unix socket. LIGHTSPEED_ENVD_LISTEN (--listen) now accepts unix:///absolute/path alongside host:port and tcp://host:port.

Changes

• The daemon creates the socket's parent directory, binds the socket and restricts it to its own user (0600).
• A socket left behind by an earlier run is replaced only when connecting to it is refused; a live listener, a socket the daemon cannot probe, or any other file at the path is left alone.
• Accept errors on the TCP and Unix listeners are logged and the listener keeps serving instead of stopping the daemon.
• environment-variables.md documents the new unix:// form.

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