build: Use option groups in configure output

Minor edits to current build flags and its help texts as well
as grouping shared and i18n options into separate option groups.

Also, validate i18n default/logic similar to how we treat other
options. `--download` isn't really intl-specific but is only used
for that purpose which is why it's grouped similarly.

PR-URL: https://github.com/iojs/io.js/pull/1533


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Steven R. Loomis <srl@icu-project.org>