MFC r306852

Incorporate a change from OpenBSD by millert@OpenBSD.org

Don't warn about valid time zone abbreviations. POSIX
through 2000 says that an abbreviation cannot start with ':', and
cannot contain ',', '-', '+', NUL, or a digit. POSIX from 2001
on changes this rule to say that an abbreviation can contain only
'-', '+', and alphanumeric characters from the portable character
set in the current locale. To be portable to both sets of rules,
an abbreviation must therefore use only ASCII letters." Adapted
from tzcode2015f.

This is needed to be able to update tzdata to a newer version