Make a separate home partition by default and detected by default

Summary:
An option in the installer to preserve an existing /home partition--working in conjunction with creating a separate /home by default during installation, too.

Rationale:
Many users prefer to keep a separate /home partition in case they either have to reinstall or they want to reinstall (not everyone likes upgrades). Even experienced users can accidentally put a check (or tick) mark on the wrong partition for reformatting, but especially beginner users may not know how to preserve their /home partition during a reinstallation.

Scope and Use Cases:
Nicola asked a lot of questions on the forums before she installed Ubuntu. She did a lot of Google searching too. After much consideration, she decided to take others' advice and create a separate /home partition so that her settings and personal files would be preserved in case she ever reinstalled Ubuntu.

She had been using Dapper for a long time and decided moving to Gutsy would be... gutsy. But upgrading from Dapper to Edgy to Feisty and then to Gutsy would take forever, even on her broadband connection. So she decides for a reinstall. Amazingly, the installer says, "You seem to have a /home partition as /dev/hda2. Would you like to preserve its contents and have Ubuntu installed to /dev/hda1?" She says "Yes," of course, and is glad the installer was smart enough to recognize that.

Implementation Plan:
Part of Ubiquities scanning of partitions would have to scan the contents of the partitions and not just their sizes, locations, and filesystem types. It would look for any partition whose top-level directory was /home and that had two levels down from it something like .dmrc.