[v3] char_traits fixups round two

[David Edelsohn]

	* 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