[v3] char_traits fixups round two

[Richard Earnshaw]

> 	* src/fstream.cc: New. Add _M_underflow_common specializations.
> 
> Did this file not get committed?  I cannot find it in my GCC libstdc++v3
> sources and bootstrap is failing with:
> 
> make[4]: *** No rule to make target `fstream.cc', needed by `fstream.lo'. Stop.
> 
> Thanks, David

I think not.  The regression tester has been screaming about it all night.

R.