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.