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

@srl295  With these latest additions, `configure_intl` is starting to become a bit bloated, and I'm concerned that it is already difficult to read and debug. I would like to see it slightly refactored in smaller logical chunks (functions). I don't know yet when would be the right time to do this. Now would certainly be ideal, but I don't want to make it prevent us from merging the rest. At the same time, I'd rather have it done sooner than later.