Macro syscall

macro_rules! syscall {
    ($fn: ident @FD ( $($arg: expr),* $(,)* ) ) => { ... };
    ($fn: ident @NON_FD ( $($arg: expr),* $(,)* ) ) => { ... };
    ($fn: ident @RAW ( $($arg: expr),* $(,)* ) ) => { ... };
}

Do syscall and return Result<T, std::io::Error> If use syscall@FD or syscall@NON_FD, the return value is wrapped in MaybeFd. The MaybeFd is designed to close the fd when it is dropped. If use syscall@RAW, the return value is raw value. The requirement to explicitly add @RAW is to avoid misuse.