Hello,

At first thanks to davexunit, civodul and mthl (from #guix channel) for
the ideas for this patch and for the patience in explaining me these
ideas.

The main changes are:

- Now default buffer names look like this:

    *Guix Package List: guix-profile*
    *Guix Package Info: test4*

  i.e., a buffer name contains a name of a profile used by this buffer.

- All commands for displaying generations and packages now can be called
  with "C-u".  This allows to select a profile just for this command.
  (The current profile can be changed with
  "M-x guix-set-current-profile").

Also as prefix argument was previously used in "M-x guix-generations" to
select last N generations, I added a new command "M-x
guix-last-generations".