A couple of fixes/improvements discovered in bug reports opened by myglc2. Patches:
[PATCH 1/3] emacs: Add 'M-x guix-installed-{user/system}-packages'.
As proposed at <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=22600#21>.
[PATCH 2/3] emacs: Set 'guix-buffer-item' before displaying entries.
This is a required change for the next patch.
[PATCH 3/3] emacs: Do not allow to modify system profiles.
