pull: Use `download-to-store'.

* guix/scripts/pull.scm (download-and-store): Remove.
  (unpack): Use `download-to-store' instead.

pull: Switch to the cgit URL.

* guix/scripts/pull.scm (%snapshot-url): Switch to the cgit URL, given
  that the Hydra one is not currently available.

ui: Gracefully report failures to connect to the daemon.

* guix/store.scm (&nix-connection-error): New condition type.
  (open-connection): Translate `system-error' during the `connect' call
  into `&nix-connection-error'.
* guix/ui.scm (call-with-error-handling): Add case for `nix-connection-error?'.
* guix/scripts/package.scm (guix-package): Move `open-connection' call
  within `with-error-handling'.
* guix/scripts/pull.scm (guix-pull): Likewise.
* guix/scripts/download.scm (guix-download): Move body within
  `with-error-handling'.

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>.

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.