enable "strict mode" in wesnoth lua

Enables an ilua feature called "strict mode" in all of our lua
environments. This change causes lua to report an error if a global
variable is used before it is assigned. The benefits of this are:
- Improved maintainability of lua-based add-ons, since you get
better error reporting in case of a typo.
- Improved behavior of the lua interpreter console, since mistyped
or gibberish lines resolve to an error rather than "nil", which
the console treats as a legal return value.

It is possible to disable this or work around it in individual
scripts. For more info see release notes.