[tz] zic tweak to warn about non-ASCII in filenames

[Paul Eggert]

Jonathan Leffler wrote:
> I don't see the digits in there; are they handled separately?

No, because the Theory file does not consider digits to be benign. 
Because the behavior of a TZ setting like "PST8" is specified by POSIX, 
it's problematic to use 'zic' to define a zone name like that.

> What is the benefit of the parentheses around the string?

I thought it'd help GNU Emacs indent it.  But it can be reworded to 
avoid that; I plan to do that as part of my next proposed patch.