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

Should we add a "download-icu" command line option for vcbuild.bat too? It would be consistent with how configure works, and when other deps move to this model we wouldn't have to download either all the deps or none.