* emacs/guix-main.scm (system-generation-boot-parameters)
(system-generation-param-alist, system-generation-sexps): New procedures.
(entries): Add 'system-generation' entry type.
* emacs/guix-messages.el (guix-result-message): Use the same messages
for 'generation' and 'system-generation' entry types.
* emacs/guix-ui-system-generation.el: New file.
* emacs.am (ELFILES): Add it.
* doc/emacs.texi (Emacs Commands): Document new commands.
* NEWS: Mention new interface.
* emacs/guix-main.scm (generation-package-specifications): Rename to...
(profile-package-specifications): ... this. Take a single 'profile'
argument.
(generation-difference): Rename to...
(profile-difference): ... this. Take profiles as arguments.
(package/output-sexps): Adjust accordingly.
* emacs/guix-ui-generation.el (guix-generation-list-profiles-to-compare):
New procedure.
(guix-generation-list-show-added-packages)
(guix-generation-list-show-removed-packages): Use it.
* emacs/guix-messages.el (guix-messages): Replace 'generation-diff' with
'profile-diff'.
(guix-message-outputs-by-diff): Adjust accordingly.