[OpenAFS] Mount points for .backup volumes

[Nickolai Zeldovich]

> On the windows client, a colleague was also faced with a directory listing
> along the lines of
> 
>     \.backup\.backup\.backup\.backup\.backup\.backup\.backup\.backup\...

This certainly seems like a bug in the windows client -- the unix
cache manager refuses to evaluate a backup volume mountpoint in
a backup volume, returning ELOOP.

(FWIW, in the athena.mit.edu cell, backup user volumes are mounted
as ~/OldFiles by default, and it seems to work fairly well..)

-- kolya