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'.
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