fwcontrol update

[Dieter]

> >> len = read(fd, buf, 1024*16);
> >
> > It would be good practice to check for read failing.
>
> Indeed it would.  I looked into fwdev.c::fw_read() a little bit and 
> documented what I think it's doing as well.

> BTW, did the compile warnings on AMD64 go away? 

I still need to give it a closer inspection, and run some tests,
but it looks good so far.  And gcc isn't complaining (7.0 on AMD64).