Re: new module suggestion: fprintftime-check

[Paul Eggert]

> I suspect that changing this behavior would be a disruptive
> backwards-incompatible change (but other opinions are welcomed).

I wouldn't mind a change as long as it changes the API enough so that compilers 
complain if we don't also update the calling code. For example, nstrftime could 
take an additional ptrdiff_t * argument that (if not null) is set to the offset 
of the first offending % or encoding error, or to -1 if the format is OK. (We 
should report encoding errors as well as bad % formats.)

(Using ptrdiff_t is part of my campaign to prefer ptrdiff_t to size_t. While 
we're at it, let's change the other size_t args to ptrdiff_t, but I digress....)

> nstrtime's __strftime_internal() is a complicated beast, I don't
> claim to fully understand it.
> But the function contains this comment:
>     /* POSIX.1 requires that local time zone information be used as
>       though strftime called tzset.  */
> https://git.savannah.gnu.org/cgit/gnulib.git/tree/lib/nstrftime.c#n540

This comment is for the POSIX API where there was no timezone_t argument. It 
doesn't apply to the Gnulib API, because of the "undef HAVE_TZSET" earlier on. 
The only reason for the tzset_called stuff is the faint hope that we can merge 
all this stuff into glibc someday.