Include poll.h and shim MSG_NOSIGNAL in socket.c for non-Linux POSIX

[johnmatter]

This makes OUO build/run on Apple Silicon.

• socket.c references MSG_NOSIGNAL and uses poll() outside the Linux epoll path, but never includes <poll.h> and assumes the GNU-only MSG_NOSIGNAL flag is defined.
• macOS provides poll() via <poll.h> and suppresses SIGPIPE via setsockopt/SO_NOSIGPIPE instead, so the flag isn't defined — adding the include and a #define MSG_NOSIGNAL 0 fallback for non-Linux gets the file compiling and running cleanly.
• No behavior change on Linux from what I can tell.
• Tested by booting against a custom shard on macOS 25.4, connecting via TCP, clean shutdown.

Let me know if you have any concerns :)

[draxinar]

This looks good.

Please rename the commit message so it's prefixed by the name of the subsystem, like:

socket: include poll.h and shim MSG_NOSIGNAL in socket.c for non-Linux POSIX

[johnmatter]

done! Thanks for the quick response