Two problems with the original change

1. We should return bytes actually processed on an error according to
SuS/POSIX. Technically the EFAULT path is outside the spec but its best
we follow

2. We need to fix most of this anyway because the final section of the
change was wrong. If retval was set we retried and got an efault again
in some cases

I think this way of doing it is right but it could do with further
review