build: i18n: Autodownload ICU, Intl `small-icu` by default!

* make `--with-intl=small-icu` the default
 * Download, verify (md5), unpack ICU's zip if not there
 * update docs

This is to support joyent#7676, Intl by default.

There's a "list" of URLs being used, but right now only the
first is picked up. The logic works something like this:

* if there is no directory `deps/icu`,
 * if no zip file (currently `icu4c-54_1-src.zip`),
  * download zip file (icu-project.org -> sf.net)
 * verify the MD5 sum of the zipfile
  * if bad, print error and exit
 * unpack the zipfile into `deps/icu`
* if `deps/icu` now exists, use it, else fail with help text