Add new "Power User" install option, with dual boot support.

Also clean up post_install and preseeds to handle dual-boot scenarios better

Also enable networking for power users during install

Also switch "Expert" install option to use power user preseed (to provide networking and partitioning)

Also increase GRUB timeout from 1 second to 5 for dual-booters
Merge branch 'master' of https://github.com/directhex/steamos-installer