policy on __libc_*

[Roland McGrath]

> You are right, that would be nice.  But I really have no time to write
> this all down in detail.  I think I've explained on the list a few
> times already that the __libc_* names are introduced for cancelation
> points since the functions must be intercepted even when called from
> inside the libc.  I.e., the __* functions also have to be exported.

I did not ask why they exist, I do already know that.  I asked which
functions need them.  All libc functions that are POSIX.1 cancellation
points?  Only some of them?  Is there a rule, or are we doing it as needed?