Intl: Automatically download ICU if requested. Reduce size. Keep `--with-intl=none` the default. 

Background: see joyent#7676 of course

* [x] decide which variant is on by default
* [x] make it on by default
* [x] test more failure cases of downloading
* [x] add md5 sum
* [x] test more platforms

If we're not using it, I would remove it. Otherwise, we'll have to maintain it without a use case and testing that it still works would require manual operations that would need to be documented.