Bug in isatty (all systems!)

[hamilton at uiucuxc.Uiuc.ARPA]

> No where does the documentation explicitly say "Some routines which return
> successful results may, as a side-effect, set the value of errno to an
> arbitrary value."  In addition, I would consider such cases to warrant
> mention in the "Special Considerations" :-) section.

of course the documentation doesn't say that.  it isn't true.
isatty() and mktemp(), for example, don't even "know" errno exists.