__* in installed headers

[Roland McGrath]

> I tend to agree with Roland.
> 
> We should put a warning in the documentation about the __-prefixed
> names being for internal use only.  Probably something in the FAQ
> too.  People who do not read the documentation deserve to get
> punished.  I'm not really afraid that a lot of people will actually
> use the __-prefixed names.

Note that I do not object to finding another place for the __ prototypes
and keeping them out of the public headers.  I just want to avoid a lot of
effort and future headaches for the maintainers (i.e. us), and find the
solution Uli suggested in particular worse than leaving things as they are
now and telling users not to use them.