Let string extraction scripts also extract plural field

Now the .pot file generated by update_pot.sh will contain proper msgid_plural fields for item names.
It has been tested roughly to work with translation files.
Use nname() instead of name in item wish menu

nname() is the function to get an item type’s name with respect
of grammatical number. Warning: Using name only returns the
English name now!
Add plurals to “EW pack” and “Oldies Pack” mods