[v3] more istream::ignore cleanups

[Paolo Carlini]

Benjamin Kosnik wrote:

>Yo Paolo, happy new year.
>
>I think you can be a bit more aggressive about the istream::ignore
>cleanups. Let me know what you think of the following patch.
>
>Also, I am mystified by:
>
>		  if (__n == numeric_limits<streamsize>::max()
>		      && !traits_type::eq_int_type(__c, __eof)
>		      && !traits_type::eq_int_type(__c, __delim))
>		    _M_gcount = numeric_limits<streamsize>::min();
>
>this resetting of _M_gcount. Any chance you can explain?
>  
>
Hey, you want to undo an effort of a few days!!!

Of course I can explain!

The problem is, on 32-bit machines, LFS enabled, your file can be bigger 
that 2G, thus more than 2G chars to ignore, in general, whereas 
streamsize, being basically a ptrdiff_t, is still limited to 32-bit: we 
are at risk of *signed* integer overflow, which is absolutely 
verboten!!! Still, we want to allow the user to keep on using ignore, in 
the general, unlimited form, also on such platforms...

If you want, I can add comments...

Paolo.