Bug in isatty (errno setting/clearing)

[Peter da Silva]

> I got out MY V6 manuals and the following is a quote from the getc(III)
> 
> DIAGNOSTICS
> 	c-bit is set on EOF or error; from C, negative return indicates
> 	EOF.  Moreover, 'errno' is set by this routine just as it is for
> 	a system call (see introduction (II)).
> 
> Also putc, putw, fcreat, fflush, all happen to munge errno, as necessary.
> 
> So in V6, we have errno munged by the library routines in section 3.

And in V6 putc didn't wreck arguments with side-effects like V7 putc does,
because it's a macro. It'd be hard to ensure the behaviour of errno in a
macro, I would think. At the least it'd slow things down (losing a bit of why
you made it a macro in the first place). Also is this from libc or libS?