Function spawn

pub fn spawn(
    name: Option<&str>,
    path: &str,
    argv: Vec<&str>,
    flags: SpawnFlags,
    priority: Option<ContextPriority>,
    stdin: Option<Ri>,
    stdout: Option<Ri>,
    stderr: Option<Ri>,
) -> Result<Pid, ErrorStatus>

same as unsafe_spawn but safe because it makes it clear that argv are consumed