Modernize zic's test for valid timezone abbreviations.

We really need to sync all of our IANA-derived timezone code with upstream,
but that's going to be a large patch and I certainly don't care to shove
such a thing into stable branches immediately before a release.  As a
stopgap, copy just the tzcode2016c logic that checks validity of timezone
abbreviations.  This prevents getting multiple "time zone abbreviation
differs from POSIX standard" bleats with tzdata 2014b and later.