Prefer using `RawSyscall` for non-blocking calls of `ioc`.

[sergiu128]

Introduced by #136. We'll have an option ioc.SetBusyWait() which, if on, ensures all syscalls are done with syscall.RawSyscall. We'll probably need to do something like:

func Read() (int, error) {
    if ioc.BusyWait() { return syscall.RawSyscall(...) } else { return syscall.Syscall(...) }
}