Main server ja translation on manual : prioritize installer .po

To: Debian Edu team
CC: Debian ja translators (debian-Japanese ML)

Greeting.

I've considered about "main server" ja candidates
for Debian Edu Buster manual during my translation.

There are a problem of mismatched translation.

Current Situation
=================

* installer uses "主サーバ"@ja = "main server"@en

#. Type: multiselect
#. Choices
#. __Choices: Main-Server, Workstation, Roaming-Workstation, 
LTSP-Server, Standalone, Minimal
#: ../debian-edu-profile-udeb.templates:2001
msgid "Main Server"
msgstr "主サーバ"

* the manual uses "中心サーバ"@ja != "main server"@en.
   It rather means "center/central server"@en.

   I don't know the reason.
   Some Japanese prefer the manual expression,
   Some other prefer the installer expression.

My suggestion
=============

Unify those terms "主サーバー"@ja or "メインサーバー"@ja,
both means "main server"@en; it's just differences between
katakana and kanji.

IMHO "メインサーバー"@ja (= "主サーバ"@ja.installer.po) would be
better, but also think changing installer.po so soon
may make things more complicated.

So, at first I'll attempt to unify them as "主サーバー"@ja in
both installer (and other components) po file(s) and the manual.

(add omitted prolonged sound symbol (I'll call it OPSS) at its tail;
  more natural expression in "2019" Japan)

Reasons
-------

* The installer .po has longer history.

* "main server" is only one in Debian Edu system.

* "main server" could have "standby server" for fault tolerance
   and/or load balance purpose in the future (as an option), I hope.

   I guess it may make the Debian Edu system more widely used.

   In that occasion,
   "主サーバー" allows terms like "standby server" translation easier.
   e.g.) "副サーバー"@ja and/or "代替サーバー"@ja.

Comments
---------

I've not noticed about installer Japanese display,
since I usually prefer expert mode installation in English.

Summary
========

* I'll try to create and send a diff patch for installer .po for OPSSs.

   I should make them for the other components .po files, too.

* I'll edit the manual to change "中心サーバ"@ja
   into "主サーバー"@ja.installer.po-with-OPSS-patch.

   The reason is described above. More detail, please read the thread
   https://lists.debian.org/debian-edu/2019/05/msg00020.html

If you have any suggestion, please tell me.

Regards.