Better support for single-user systems

[Taylan Kammer <taylan.kammer@gmail.com>]

Most desktop users have single unix account and are also in control of
root.  These users might not want to differentiate between the current
guix version of root and their normal user.  They might also not want
to differentiate between the packages available to root and the normal
user.  As such I would propose the following two improvements:

- Allow a system-wide guix installation that's updated with a special
  variant of 'guix pull' executed by root

- Allow direct addition of packages to the system profile to obviate
  the need of running a full 'guix system reconfigure' after adding
  packages to the system configuration

(The latter might show a reminder that if the package isn't also added
to the system config, it will be removed again on the next system
reconfiguration.)

Currently I use a hack to imitate #1 where I have a unix account
called 'guix-user' with which I run 'guix pull', and both root and my
normal user have symlinks to that user's current guix.  For #2 I don't
have a workaround; I just re-run 'guix system reconfigure' every time.


What do others think?


- Taylan