[Patch] Move streambuf::_M_mode to filebuf and stringbuf

[Paolo Carlini]

Nathan Myers wrote:

>>@@ -214,6 +221,7 @@
>>       {
>> 	this->close();
>> 	_M_buf_size = 0;
>>+	_M_mode = ios_base::openmode(0);
>> 	_M_last_overflowed = false;
>>       }
>>    
>>
>One question -- I know you just copied what was here, but -- why 
>are we re-initializing data members in the destructor?   Let's
>trash all three assignments.  
>
You are right, of course. I was also preparing something else and in the 
rush didn't consider attentively the issue. Will do the change and 
commit tomorrow.

Paolo.