Avoid <ctype.h> locale problems.

* zdump.c, zic.c: Don't include <ctype.h>, because the behavior if
its macros is locale-dependent if HAVE_GETTEXT, and we want the C
locale's semantics.  Instead, use portable replacements for
ctype.h operations, with the desired semantics.  All uses of
isalpha etc. replaced by calls to new functions is_alpha etc.  or
by inline code.
(isascii): Remove.
(is_alpha): New function.
* zic.c (doabbr): Simplify by using is_alpha.
(is_space): New function.
* NEWS: Document the above.