Add --enable-fhs flag to configure

When --enable-fhs is passed to configure, we set all paths by default
in a way compliant with FHS, the Filesystem Hierarchy Standard.

  http://www.pathname.com/fhs/

Each path may still be overridden by passing the specific flag for it.
Update the configuration report

Order the default configure flags in the order they show up in
configure --help, and order our own alphabetically.