guix gc: Add `--references' and `--referrers'.

* guix/scripts/gc.scm (show-help): Update.
  (%options): Add `--references' and `--referrers'.
  (guix-gc)[symlink-target, store-directory]: New procedures.
  Handle the `list-references' and `list-referrers' actions.
* tests/guix-gc.sh: Add tests for `--references'.
* doc/guix.texi (Invoking guix gc): Document `--references' and
  `--referrers'.

store: Add queries for references & co.

* guix/store.scm (operation-id)[query-valid-derivers]: New value.
  (references, referrers, valid-derivers, query-derivation-outputs): New
  procedures.
* tests/store.scm ("references", "derivers"): New tests.

download: Add X.org mirrors.

* guix/download.scm (%mirrors): Add `xorg'.

gnu: screen: Use GNU mirror.

* gnu/packages/screen.scm (screen): Use GNU mirror.

gnu: Update libpng to 1.5.14.

* gnu/packages/libpng.scm (libpng): Switch to version 1.5.14.

pull: Distinguish "already up to date" from "updated".

* guix/ui.scm (show-what-to-build): Return (length req*).
* guix/scripts/pull.scm (guix-pull): Print an "already up to date"
  message when there's nothing to build.

pull: Build (guix build download) first, because of the (gnutls) autoload.

* guix/scripts/pull.scm (unpack): Build (guix build download) first, as
  done in 855a8ad71def2ebc594ed32c57bda0ca4e13d91c.
  Reported by Andreas Enge <andreas@enge.fr>.

guix: Make sure UPDATES-DIR is valid.

* scripts/guix.in: Make sure UPDATES-DIR is not #f.

Add "guix pull".

* guix/scripts/pull.scm: New file.
* Makefile.am (MODULES): Add it.
* doc/guix.texi (Invoking guix pull): New node.
  (Invoking guix package): Add cross-ref to it.
* guix/ui.scm (config-directory): New procedure.
* scripts/guix.in: When `GUIX_UNINSTALLED' is undefined, add
  $XDG_CONFIG_HOME/guix/latest to the search path.
* po/POTFILES.in: Add guix/scripts/pull.scm.

ui: Factorize `show-what-to-build'.

* guix/scripts/package.scm (guix-package)[show-what-to-build]: Move to..
* guix/ui.scm (show-what-to-build): ... here.  Add a `store'
  parameter'.  Adjust callers.
* guix/scripts/build.scm (guix-build): Use it.  Remove `req' and `req*'
  variables.

build: Adjust guix.texi to Texinfo 5.0.

* doc/guix.texi: Change @title and @subtitle syntax to please Texinfo 5.0.

ui: Add temporary file handling and atomic symlink switch.

* guix/scripts/download.scm (call-with-temporary-output-file): Move to
  ui.scm.
* guix/scripts/package.scm (switch-symlinks): Likewise.
* guix/ui.scm (call-with-temporary-output-file, switch-symlinks): New
  procedures.

download: Adjust to `http-get*' deprecation.

* guix/build/download.scm (http-fetch): Adjust to use #:streaming? when
  using Guile 2.0.8+.

derivations: Add a search path parameter for module derivations.

* guix/derivations.scm (imported-modules, compiled-modules): Add a
  `module-path' parameter.  Use it instead of %LOAD-PATH.

gnu: texinfo: Make Perl a propagated input.

* gnu/packages/texinfo.scm (texinfo): Make PERL a propagated input.

build: Add missing -I flag for the daemon.

* daemon.am (libstore_a_CPPFLAGS): Add missing -I, for schema.sql.hh.

daemon: Add `--listen'.

* nix/nix-daemon/guix-daemon.cc (GUIX_OPT_LISTEN): New macro.
  (options): Add `--listen'.
  (parse_opt): Handle it.
* doc/guix.texi (Invoking guix-daemon): Mention it.

Update from upstream Nix.

store: Add the `%daemon-socket-file' parameter.

* guix/store.scm (%daemon-socket-file): New variable.
  (open-connection): Use it as the default value for FILE.

Add Mark to `AUTHORS'.