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.