emacs: config: Use "emacs-config-" prefix for constants.

* emacs/guix-config.el.in (guix-emacs-interface-directory,
  guix-state-directory): Rename to ...
  (guix-config-emacs-interface-directory, guix-config-state-directory):
  ...this.
  (guix-config-guile-program): New constant.
  (guix-guile-program): Move to ...
* emacs/guix-external.el (guix-guile-program): ...here.  Make it a
  'defcustom'.
* emacs/guix-profiles.el (guix-default-profile): Use
  'guix-config-state-directory'.
* emacs/guix-backend.el (guix-load-path): Use
  'guix-config-emacs-interface-directory'.

build: Set DOT_USER_PROGRAM for Emacs interface.

Suggested by Ludovic Courtès <ludo@gnu.org>.

* configure.ac: Set DOT_USER_PROGRAM variable.
* emacs/guix-config.el.in (guix-config-dot-program): New constant.
* emacs/guix-external.el (guix-dot-program): Use it.

gnu: units: Update to 2.12.

* gnu/packages/maths.scm (units): Update to 2.12.

Add 'guix challenge'.

* guix/scripts/challenge.scm, tests/challenge.scm: New files.
* Makefile.am (MODULES): Add the former.
  (SCM_TESTS): Add the latter.
* doc.am (SUBCOMMANDS): Add 'challenge'.
* doc/guix.texi (Substitutes): Add xref to 'guix challenge'.
  (Invoking guix challenge): New node.
* doc/contributing.texi (Submitting Patches): Add note about using 'guix
  challenge'.
* po/guix/POTFILES.in: Add guix/scripts/challenge.scm.

substitute: Expose narinfo access.

* guix/scripts/substitute.scm: Export <narinfo> accessors.
  (narinfo-hash->sha256): New procedure.
  (cache-narinfo!): Ignore EACCES exceptions.

guix archive: Add '--extract'.

* guix/scripts/archive.scm (show-help, %options): Add --extract.
  (guix-archive): Honor it.
* tests/guix-archive.sh: Test it.
* doc/guix.texi (Invoking guix archive): Document it.

gnu: elogind: Provide the file name of 'halt' and 'reboot'.

* gnu/packages/freedesktop.scm (elogind)[inputs]: Add DMD.

gnu: gnurl: Update to 7.44.0.

* gnu/packages/gnunet.scm (gnurl): Update to 7.44.0.

gnu: calibre: Update to 2.41.0.

* gnu/packages/ebook.scm (calibre): Update to 2.41.0.

gnu: youtube-dl: Update to 2015.10.16.

* gnu/packages/video.scm (youtube-dl): Update to 2015.10.16.

gnu: ngs-sdk: Correct typo.

* gnu/packages/bioinformatics.scm (ngs-sdk): Correct typo.

gnu: ngs-sdk: Remove unsupported systems.

* gnu/packages/bioinformatics.scm (ngs-sdk)[supported-systems]: Keep only x86
  based systems.

gnu: chess: Update to 6.2.2.

* gnu/packages/games.scm (chess): Update to 6.2.2.

gnu: non-sequencer: Disable SSE when not building on x86_64.

* gnu/packages/music.scm (non-sequencer)[arguments]: Add "--disable-sse"
  flag when not building on x86_64.

gnu: fastcap: Remove non-free file.

* gnu/packages/engineering.scm (fastcap): Do it.

gnu: fastcap: Fix 'fix-doc phase.

* gnu/packages/engineering.scm (fastcap): Adapt documentation generation to
  change in TeXLive 2015.

emacs: devel: Highlight Guix keywords.

* emacs/guix-devel.el (guix-devel-keywords): New constant.
  (guix-devel-font-lock-keywords): Use it.

doc: emacs: Document "View graph" popup action.

* doc/emacs.texi (Emacs Popup Interface): Document "View graph" action.

emacs: Improve file names of generated graphs.

Suggested by Ludovic Courtès <ludo@gnu.org>.

* emacs/guix-external.el (guix-png-file-name): Change prefix of a file
  name to "guix-emacs-graph-" to avoid possible conflicts.

gnu-maintenance: Fix file descriptor leak.

* guix/gnu-maintenance.scm (official-gnu-packages): Close the port
  returned by FETCH.