Name module "init" instead of "__namespace"

See discussion at https://gerrit.wikimedia.org/r/#/c/144447/.

I'm open for discussion about the name `init`. Could also be `namespace`. My problem are the underscores. This is not a magic keyword (compare to PHP's `__LINE__` and MediaWiki's `__NOTOC__`).

If you merged this please also merge https://gerrit.wikimedia.org/r/#/c/146185/.