NativeCall::Errno

Use NativeCall::Errno to read errno from Perl 6 code after invoking native
functions. For example:

    sub pipe(CArray[int32] --> int32) is native {*}

    my @ends := CArray[int32].new;
    @ends[0] = @ends[1] = 0;
    if pipe(@ends) == -1 {
        errno.fail;
    }
