[v3] Fix second half of libstdc++/6745

[Paolo Carlini]

Jonathan Lennox wrote:

>>I want to study this in more detail but basically I believe you are 
>>right: we have a performance issue in such cases. Could you possibly work
>>out a patch along the lines which you suggested?
>>Not adding new public members to basic_filebuf, however...
>>    
>>
>Is making __copy_streambufs a friend of basic_filebuf acceptable?
>
Probably...

>Also, I'm still not clear on whether it's acceptable to use dynamic_cast<>
>in libstdc++.
>
Nice question... In fact, in the current codebase there aren't, I'm not 
sure why, however.

Benjamin?

Ciao, Paolo.