emacs: Handle autoloading Emacs packages installed with Guix.

* emacs/guix-emacs.el: New file.
* emacs.am (ELFILES): Add it.
* emacs/guix-backend.el (guix-after-repl-operation-hook): Add
  'guix-emacs-load-autoloads-maybe'.
* emacs/guix-init.el.in: Autoload emacs packages.
  (guix-package-enable-at-startup): New variable.

emacs: Move profiles code to "guix-profiles.el.in".

* emacs/guix-base.el (guix-user-profile, guix-default-profile,
  guix-current-profile, guix-profile-prompt, guix-set-current-profile): Move
  to...
* emacs/guix-init.el.in (guix-default-profile): Move to...
* emacs/guix-profiles.el.in: ... here.  New file.
* configure.ac (AC_CONFIG_FILES): Add "emacs/guix-profiles.el".
* emacs.am (nodist_lisp_DATA): Likewise.

emacs: Add 'guix-prettify'.

* emacs/guix-prettify.el: New file.
* emacs.am (ELFILES): Add it.
* doc/emacs.texi (Emacs Prettify): New node.
* doc/guix.texi (Features): Mention it.

build: Add missing "emacs/guix-messages.el".

* emacs.am (ELFILES): Add "emacs/guix-messages.el".

build: Don't install elisp files when Emacs is unavailable.

* emacs.am (dist_lisp_DATA, nodist_lisp_DATA): Wrap in 'if HAVE_EMACS'.

build: Don't generate Emacs autoloads when Emacs is unavailable.

Reported by Andreas Enge.

* configure.ac: Define 'HAVE_EMACS' Automake conditional.
* emacs.am ($(AUTOLOADS)): Wrap in 'if HAVE_EMACS'.

Merge branch 'master' into core-updates

Add Emacs user interface.

* configure.ac (emacsuidir): New variable.
  (AC_CONFIG_FILES): Add 'emacs/guix-init.el', 'emacs/guix-helper.scm'.
* Makefile.am: Include 'emacs.am'.
* emacs.am: New file.
* doc/emacs.texi: New file.
* doc/guix.texi: Include 'emacs.texi'.
* emacs/guix-backend.el: New file.
* emacs/guix-base.el: New file.
* emacs/guix-helper.scm.in: New file.
* emacs/guix-history.el: New file.
* emacs/guix-info.el: New file.
* emacs/guix-init.el.in: New file.
* emacs/guix-list.el: New file.
* emacs/guix-main.scm: New file.
* emacs/guix-utils.el: New file.
* emacs/guix.el: New file.