emacs: Add support for modifying options during operation confirmation.
* emacs/guix-base.el (guix-operation-option-key): New face.
(guix-operation-option-true-string, guix-operation-option-false-string,
guix-operation-option-separator, guix-operation-options): New variables.
(guix-operation-option-by-key, guix-operation-option-key,
guix-operation-option-name, guix-operation-option-variable,
guix-operation-option-value, guix-operation-option-string-value,
guix-operation-prompt, guix-operation-set-mode-line): New procedures.
(guix-continue-package-operation-p): Use 'guix-operation-prompt'.
b497a85be8