Dreadful gmirror performance - suggested changes to 'prefer'

[Pete French]

> Hmm, it would seem you need "N-and-upper" and "N-and-lower", but this is
> inconvenient. Your original idea is probably better.

Certainly simpler to implement. Ideally, of course, you could change the
priority on the fly (which would solve all of this) but the fact that it
is stored in priority order makes that a bit of a pain too I would imagine.

Do you have any thoughts on dumping by the way ? I initially implemnted the
code to chheck for 'prefer-low' and dump to the last disc instead of the first.
But the manual page suggests ways of working with prefer which may well be
broken doing it that way, so I took the change out in order to make the
manpage advice still correct.

I have a working implementation here now if anyone wants to test by the
way - I ended up simply adding a 'prefer-low' and changing the default
priority. Everything else behaves the same as it always did.

-pete.