emacs: Improve interface for working with multiple profiles.

Suggested by David Thompson, Ludovic Courtès and Mathieu Lirzin.

* emacs/guix-base.el (guix-profile-prompt): New procedure.
  (guix-set-current-profile): Use it.
  (guix-buffer-name-simple, guix-buffer-name-default, guix-buffer-name): New
  procedures.
  (guix-buffer-name-function, guix-profile): New variables.
  (guix-set-vars, guix-get-entries, guix-get-show-entries, guix-set-buffer,
  guix-history-call, guix-process-package-actions,
  guix-continue-package-operation-p, guix-delete-generations,
  guix-switch-to-generation): Add 'profile' argument.
* emacs/guix.el (guix-get-show-packages, guix-get-show-generations,
  guix-search-by-name, guix-search-by-regexp, guix-installed-packages,
  guix-obsolete-packages, guix-all-available-packages,
  guix-newest-available-packages, guix-generations, guix-generations-by-time):
  Likewise.
  (guix-last-generations): New command.
* emacs/guix-info.el: Adjust for using 'profile' argument where needed.
* emacs/guix-list.el: Likewise.
* doc/emacs.texi (Emacs Commands): Document 'guix-last-generations' and using
  "C-u" for commands.
  (Emacs Buffer Names): Document 'guix-buffer-name-function'.