Implement Xsrandomdev for darwin

srandomdev() seeds the random number generator from /dev/urandom. The previous stub panicked with todo(""), breaking any program that calls it on macOS (e.g. SQLite AFP locking path on network volumes). I discovered this was an issue from investigating https://github.com/charmbracelet/crush/issues/3171 and eventually traced it through modernc.org/sqlite to here.