fix code problems found by luacheck
Second iteration of the process, now handling data/lua/wml/*.lua
luacheck command used to find bugs:
luacheck ./*.lua --globals wesnoth wml --codes --ignore 542 213
Additionally, error code 211 (unused variables) could be ignored,
as using underscore convention `_` is controversial in
wesnoth ( see https://github.com/wesnoth/wesnoth/pull/2380#discussion_r162519341 )
Actual bugs found:
* items.lua, access of global `write_name` instead of local `cfg.write_name`
* kill.lua, typo `primary_unit` -> `primary`
* bad code style: global `i` instead of local `i`
(would conflict with 3-rd party code if it would use global `i`, too)
