guix-package: Add `--list-available'.

* guix-package.in (show-help, %options): Add `--list-available'.
  (guix-package)[process-query]: Add support for `--list-available'.
* doc/guix.texi (Invoking guix-package): Document it.
* tests/guix-package.sh: Add test.

* guix/ui.scm (location->string): New procedure.
* guix/utils.scm: Export <location>.

ui: Factorize `--version'.

* guix/ui.scm (show-version-and-exit): New procedure.
* guix-build.in (show-version): Remove.
  (%options)["version"]: Use `show-version-and-exit'.
* guix-download.in: Likewise.
* guix-package.in: Likewise.

Add (guix ui).

* guix/ui.scm: New file.
* Makefile.am (MODULES): Add it.
* po/POTFILES.in: Add it.

* guix-build.in: Use it.
  (_, N_, leave): Remove.
  (guix-build): Use `with-error-handling' instead of the `guard' form.
* guix-download.in: Use it.
  (_, N_, leave): Remove.