Switch back to ASCII for symbols.

Garrett Wollman reported privately that XEmacs 21.4.22, the current
stable version, doesn't work with the UTF-8 recently introduced
into our commentary.  For example, the UTF-8 character '−'
(MINUS SIGN), which is stored as the three bytes "\342\210\222",
displays as 'â\210\222'.  For proper names this is annoying but
tolerable, as there's little loss in utility from (say) 'Racoviță'
to its display form 'RacoviÈ\233Ä\203'.  But for symbols this is a
real pain that can make it hard to understand the documentation, e.g.,
'Release 2014e – 2014-06-12 21:53:52 −0700' displays as
'Release 2014e â\200\223 2014-06-12 21:53:52 â\210\2220700'.
To work around this problem, make the following substitutions in
commentary to mostly revert these symbols to their pre-UTF-8 versions:
'§' -> 'section', '°' -> 'degrees', '±' -> '+-', '–' -> '-' (en
dash), '—' -> '--' (em dash), '′' -> "'", '″' -> '"', '→' -> '->',
'−' -> '-' (minus sign), '≤' -> '<='.  Leave proper names and
foreign words in UTF-8.
* tz-link.htm: Avoid non-ASCII char in source.
Add encoding directives for unibyte GNU Emacs.

(Thanks to Garrett Wollman for reporting the problem.)
* NEWS, Theory, northamerica: Add "coding: utf-8" directives so
that GNU Emacs, when run in a unibyte locale, does not mistakenly
interpret these files to be unibyte.  This change should make no
difference if you use Emacs to read these files in a UTF-8 locale.
I didn't bother to add coding directives to files where GNU Emacs
already deduces the encoding correctly in a Latin-1 locale.
Further updates to commentary, mostly un-ASCII-fying contributors' names.

Change attributions for contributors' names using the Latin alphabet
only where it was possible to reconstruct from mailing list records.
Leave attributions for Андрей Чернов, Иофис Герман Романович, and 李耀
宗 with their romanized versions instead (Andrey A. Chernov, German
Iofis, and Lee Yiu Chung, respectively).
* africa: A few fixes to typos and capitalization in commentary.
More spelling and accent fixes.
More spelling and accent fixes.

(Thanks to Ian Abbott for catching some stray '’'s.)
Improve checking for non-ASCII characters.

* Makefile (NONSYM_CHAR, NONSYM_LINE): New macros.
(SAFE_SHARP_LINE): Don't let comments contain troublesome symbols.
(check_character_set): Don't let troublesome symbols appear in
source files, and don't let non-ASCII appear in .html files.
Remove usno* files.

(Thanks to Tim Parenti for the suggestion.)
* Makefile (MISC), NEWS: Remove usno* files.
* usno1988, usno1989, usno1989a, usno1995, usno1997, usno1998: Remove.
* africa, asia, australasia, tz-link.htm: Remove commentary about
these files.
* africa, asia: Provide dates of reference for USNO citations.
Remove no-longer-necessary trademark notices.

(Thanks to Arthur David Olson.)
* private.h, zic.c: Remove UNIX notices.
Check POSIX file name restrictions more carefully.

Also, document the longstanding exceptions to POSIX.
Derived from Arthur David Olson's patch in:
http://mm.icann.org/pipermail/tz/2014-June/021052.html
* Theory: Document the exceptional file name components in 'etcetera'
and 'backward' that contain '+' or digits, or are too long.
* zic.8 (DESCRIPTION): Document the change to zic.
* zic.c (component_length_check, namecheck): New functions.
(dolink, writezone): Use them to check that file name components
obey the POSIX rules, and do not contain "." or "..".
* NEWS: Document the above.
Add 'Thanks' for previous change.
Go back to ASCII in *.txt files, too.

For *.txt files, UTF-8 is not that important, and avoiding it
is better for users with older or misconfigured manpage readers.
* Makefile (check_character_set): Check that *.txt files
contain only safe characters.
($(MANTXTS)): Build *.txt files in the C locale, not in en_US.utf8.
'zic -v' now warns about all '.'s in output file names.

* zic.c (componentcheck, namecheck): Warn about all '.'s in
the file name, not merely about "." and ".." file name components.
* zic.8 (DESCRIPTION), NEWS: Document this.
* Theory, NEWS: Also document EST5EDT etc. as exceptions.

Use the term "legacy names" for exceptions.
A few more spelling, accent, and commentary cleanups.
* zic.8, NEWS: Document character encoding issues better.

(Thanks to Guy Harris for reporting the problem.)
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.
* private.h, zdump.c (TZ_DOMAIN): Look at HAVE_GETTEXT,

not TZ_DOMAINDIR, when deciding whether to define.  This fixes a
recently-introduced typo.  Found by compiling with -DHAVE_GETTEXT
under GNU/Linux.