Update on some PRs I'm working on

[Pétur Runólfsson]

> - 9875 and 9178
> These are both about deficiencies for encodings > 1. They are
> not too difficult to fix, IMO. I have already the fixes for
> 9875, except the underflow bits (yesterday Pétur provided some
> testcases which I asked).

The best fix for underflow() is probably to add

if (gptr() < egptr())
  return *gptr();

at the top (of course using traits operators and handling uflow
correctly). I think the seekoff can then be avoided.
This code should work for all values of codecvt::encoding(),
as well as for pipes and other things for which seekoff
doesn't work.

Petur