gnu: Add sdl-union.

* gnu/packages/sdl.scm (sdl-union): New variable.
gnu: Add guile-sdl.

* gnu/packages/sdl.scm (guile-sdl): New variable.
gnu: Add polkit-qt.

* gnu/packages/polkit.scm (polkit-qt): New variable.
emacs: Add "c" key binding to copy a button link.

* emacs/guix-info.el (guix-info-button-map): New variable.
  (guix-info-button-copy-label): New procedure.
* doc/emacs.texi (Emacs Info buffer): Mention a "c" key binding.
  (Emacs Keymaps): Add 'guix-info-button-map'.
emacs: Restore mode-line after operation prompt.

* emacs/guix-base.el (guix-operation-prompt): Restore 'mode-line-format' after
  prompting.
emacs: Add 'guix-find-file'.

* emacs/guix-utils.el (guix-find-file): New procedure.
* emacs/guix-info.el (guix-file): Use it.
emacs: Improve 'guix-redisplay-buffer'.

* emacs/guix-base.el (guix-redisplay-buffer): Allow to display new entries.
  Restore point position.
doc: Avoid @indentedblock, which is not supported by Texinfo 4.

Reported by Mark H Weaver.

* doc/guix.texi (Security Updates): Use @quotation instead of
  @indentedblock; the latter is not supported by Texinfo 4.
doc: Add "GRUB Configuration".

* doc/guix.texi (operating-system Reference): Add xref to "GRUB
  Configuration".
  (GRUB Configuration): New node.
Thank Philip Woods.
Update NEWS.
doc: Mention 'guix environment' in "Features"; add index entries.

* doc/guix.texi (Features): Add paragraph about 'guix environment'.
  (Build Systems): Add @cindex.
  (Invoking guix environment): Ditto.
licenses: Add 'silofl1.1'.

* guix/licenses.scm (silofl1.1): New variable.
nls: Add a few more package modules for translation.

* po/packages/POTFILES.in: List a few more files.
install: Add GRUB to the set of global packages.

* gnu/system/install.scm (installation-os)[packages]: Add GRUB.
syscalls: Add 'mount-points'.

* guix/build/syscalls.scm (mount-points): New procedure.
* tests/syscalls.scm ("mount-points"): New test.
services: Add 'user-unmount-service' as an essential service.

* gnu/services/base.scm (user-unmount-service): New procedure.
* gnu/system.scm (essential-services): Use it.
* gnu/system/install.scm (cow-store-service): Mention it in comment.
doc: Mention "audio" and "video" groups in configuration template.

* gnu/system/os-config.tmpl (users): Add "audio" and "video" groups and
  explain.
gnu: libmikmod: Avoid dlopening libasound & co.

* gnu/packages/sdl.scm (libmikmod)[arguments]: New field.
services: Add 'auto-start?' field to <service>.

* gnu/services.scm (<service>)[auto-start?]: New field.
* gnu/services/dmd.scm (dmd-configuration-file): Honor it.
* gnu/system/install.scm (cow-store-service): Add 'auto-start?' field.