Status of basic_filebuf<> code

[Greg Bumgardner]

Now that I have finished a complete set of facet specializations, I would
like to exercise them, but the functionality I need to test codecvt<> is
currently #ifdef'd out in basic_filebuf<>'s underflow() and
_M_really_overflow() methods.

It also appears that there are some outstanding issues to resolve regarding
the imbue() method on this same class.

What needs to be done to finish this code?

-g.b.