Moving the Japanese Era data out of the locales

[Florian Weimer]

We have some level of Japanese Era support in strftime:

$ LC_ALL="ja_JP.utf-8" date +%Ec
平成30年03月14日 10時41分04秒
$ LC_ALL="ja_JP.utf-8" date +%EY
平成30年

(date may actually use a separate copy of the strftime code, but I 
believe it is substantially identical to the glibc implementation.)

There is going to be a transition in 2019 (bug 22964).  Having this in 
locale information is a bit problematic for (some) distributions because 
they have to update glibc for all users, on all release branches, just 
to deliver this change.

Could we synthesize this data from a dedicated file in 
/usr/share/locale, by changing the code in time/era.c?  Then 
distributions can ship this file as part of tzdata, which has a 
different update procedure from glibc.

Thanks,
Florian