Installer development

[Danny Milosavljevic]

Hi,

if you want, in order to start developing the installer, the easiest way is to 
run as NORMAL USER (not root):

$ mkdir guix-installer
$ cd guix-installer
$ git clone -b wip-installer-2 https://git.savannah.gnu.org/git/guix.git
$ cd guix
$ ./pre-inst-env guix environment guix --fallback --pure --ad-hoc guile-ncurses
[env] $ ./pre-inst-env guix system installer

The "environment --pure" makes sure that parted, mkfs etc is not available and 
you don't kill your existing system by accident.

In order to cancel the installer, press Ctrl-z and then invoke:

guix [env] $ kill -9 %1

I pushed a wip-installer-2 branch, starting from master.  Into the former I 
cherry-picked all the wip-installer-only commits and massaged them until the 
installer worked.  I hope that's OK with everyone.