[Patch] Simplify a bit __copy_streambufs

[Paolo Carlini]

Hi,

this all started with this simplification of the main 'if':

  if (__in_avail != 0 && __sbin->_M_in_cur
      && __sbin->_M_in_cur + __in_avail <= __sbin->_M_in_end)  

to

  if (__in_avail
      && __sbin->_M_in_end - __sbin->_M_in_cur >= __in_avail)

then, I noticed that basic_streambuf::showmanyc() just returns 0,
(and the overall structure of __copy_streambufs doesn't make
advantage of a possible non-trivial showmanyc() like that we
have for basic_filebuf) therefore __in_avail (i.e., in_avail())
its just _M_in_end - _M_in_cur. But, then, why call in_avail()
in the first place?
The other bits are trivial (nice ;) clean ups.

Tested x86-linux. Ok?

Paolo.

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