Add Docbook XML mode in tools/generatedocbook

tools/generatepot uses Docbook XML mode.
It replaces elements of several types with placeholder.

  eg: <command>ps aux</command>  ->  <placeholder-1>
      [aaa.xml]
        run <command>ps aux</command>.
      [aaa.pot]
        msgid "run <placeholder-1>."
        msgstr ""

Then, translatores translate strings with placeholder.

  eg: [ja.po]
        msgid "run <placeholder-1>."
        msgstr "<placeholder-1> wo jikkou."

So, tools/generatedocbook need to merge the tranlation strings,
considering Docbook format.

If not be considered, the strings containing a placeholder
element remains untranslated because msgid does not match.

  eg: [aaa.xml]
        run <command>ps aux</command>.
      [ja.po]
        msgid "run <placeholder-1>."    <- not match
        msgstr "<placeholder-1> wo jikkou."

Change-Id: Ieb32ed8bfb0b6880040e47e3700b812f81598d8a
Update ja.po based on GitHub translation review