Add the new template file based autopartitioning feature of disklabel(8)

Add the new template file based autopartitioning feature of disklabel(8)
to the OpenBSD installer. It is available during unattended installation.
The template file is fetched from an url, provided as answer to a new
question in the response file:

    URL to autopartitioning template for disklabel = url

Original diff from and OK henning@
'no objection' krw@