Unix/Linux installation: Update requirements

- Update Debian/Ubuntu docs to match what's provided by current releases (package names and versions) and update the Java installation notes to default to the least insecure and best supported option
- Update Java requirements
- Update Python requirements (minor rewording)
- Update Ice requirements
- Update environment variables

See the commit logs for further details of each.  The primary correction here is to remove `ICE_HOME` in the installation instructions given that it's neither used nor required.  It also corrects a number of inaccuracies and errors in the default set of environment variables documented to be required.  Most were useless, some are required under certain circumstances; this is all cleaned up and explained.