* util/grub-mkconfig_lib.in (gettext_quoted): Add clarifying

* util/grub-mkconfig_lib.in (gettext_quoted): Add clarifying
comment.  Add an extra layer of quotation, requiring the output of
this function to be used in a printf format string.
(gettext_printf): New function.
* util/grub.d/10_hurd.in: Use gettext_printf where appropriate.
Extract translatable strings from here-documents and use a temporary
variable instead, so that xgettext can find them.
* util/grub.d/10_kfreebsd.in: Likewise.
* util/grub.d/10_linux.in: Likewise.
* util/grub.d/20_linux_xen.in: Likewise.

* po/grub.d.sed: New file.
* po/Makefile.in.in ($(DOMAIN).pot-update): Extract gettext_printf
arguments.  Set c-format flags on all strings extracted from
util/grub.d/ (xgettext refuses to include these itself for strings
it extracted from a shell file, but these really are c-format).