[Patch] Clean up sbumpc, move back _M_buf_size, do not special case unbuffered _M_underflow...

[Benjamin Kosnik]

Paolo, didn't you reference some pbackfail tests in the earlier
discussion of this patch? I didn't see these new tests in your
ChangeLog: are they no longer relevant? Or are you just going to add
them later? 

Here's what I'm remembering:
http://gcc.gnu.org/ml/libstdc++/2003-06/msg00079.html

No biggie. Please check this in as-is.

>Also (but not so as to delay applying the patch), why do we care whether 
>the stream is_open()?

See:

27.8.1.4 - Overridden virtual functions 

If setbuf(0,0) is called on a stream before any I/O has occured on that
stream, the stream becomes unbuffered.

Both v2 and v3 assume that unopened filebufs haven't had any io.

-benjamin