ui: Rename '_' to 'G_'.

This avoids collisions with '_' when the latter is used as a 'match'
pattern for instance.  See
<https://lists.gnu.org/archive/html/guix-devel/2017-04/msg00464.html>.

* guix/ui.scm: Rename '_' to 'G_'.
* po/guix/Makevars (XGETTEXT_OPTIONS): Adjust accordingly.
* build-aux/compile-all.scm (warnings): Remove 'format'.
* gnu/packages.scm,
gnu/services.scm,
gnu/services/shepherd.scm,
gnu/system.scm,
gnu/system/shadow.scm,
guix/gnupg.scm,
guix/http-client.scm,
guix/import/cpan.scm,
guix/import/elpa.scm,
guix/import/pypi.scm,
guix/nar.scm,
guix/scripts.scm,
guix/scripts/archive.scm,
guix/scripts/authenticate.scm,
guix/scripts/build.scm,
guix/scripts/challenge.scm,
guix/scripts/container.scm,
guix/scripts/container/exec.scm,
guix/scripts/copy.scm,
guix/scripts/download.scm,
guix/scripts/edit.scm,
guix/scripts/environment.scm,
guix/scripts/gc.scm,
guix/scripts/graph.scm,
guix/scripts/hash.scm,
guix/scripts/import.scm,
guix/scripts/import/cpan.scm,
guix/scripts/import/cran.scm,
guix/scripts/import/crate.scm,
guix/scripts/import/elpa.scm,
guix/scripts/import/gem.scm,
guix/scripts/import/gnu.scm,
guix/scripts/import/hackage.scm,
guix/scripts/import/nix.scm,
guix/scripts/import/pypi.scm,
guix/scripts/import/stackage.scm,
guix/scripts/lint.scm,
guix/scripts/offload.scm,
guix/scripts/pack.scm,
guix/scripts/package.scm,
guix/scripts/perform-download.scm,
guix/scripts/publish.scm,
guix/scripts/pull.scm,
guix/scripts/refresh.scm,
guix/scripts/size.scm,
guix/scripts/substitute.scm,
guix/scripts/system.scm,
guix/ssh.scm,
guix/upstream.scm: Use 'G_' instead of '_'.  Most of this change was
obtained by running: "sed -i -e's/(_ "/(G_ "/g' `find -name \*.scm`".

guix/scripts/import/pypi.scm

@@ -38,11 +38,11 @@
   '())
 
 (define (show-help)
-  (display (_ "Usage: guix import pypi PACKAGE-NAME
+  (display (G_ "Usage: guix import pypi PACKAGE-NAME
 Import and convert the PyPI package for PACKAGE-NAME.\n"))
-  (display (_ "
+  (display (G_ "
   -h, --help             display this help and exit"))
-  (display (_ "
+  (display (G_ "
   -V, --version          display version information and exit"))
   (newline)
   (show-bug-report-information))
@@ -68,7 +68,7 @@ Import and convert the PyPI package for PACKAGE-NAME.\n"))
     ;; Return the alist of option values.
     (args-fold* args %options
                 (lambda (opt name arg result)
-                  (leave (_ "~A: unrecognized option~%") name))
+                  (leave (G_ "~A: unrecognized option~%") name))
                 (lambda (arg result)
                   (alist-cons 'argument arg result))
                 %default-options))
@@ -83,10 +83,10 @@ Import and convert the PyPI package for PACKAGE-NAME.\n"))
       ((package-name)
        (let ((sexp (pypi->guix-package package-name)))
          (unless sexp
-           (leave (_ "failed to download meta-data for package '~a'~%")
+           (leave (G_ "failed to download meta-data for package '~a'~%")
                   package-name))
          sexp))
       (()
-       (leave (_ "too few arguments~%")))
+       (leave (G_ "too few arguments~%")))
       ((many ...)
-       (leave (_ "too many arguments~%"))))))
+       (leave (G_ "too many arguments~%"))))))