DRAFT system: hurd: Remove bash from boot process.

Guile needs pipe support for its finalizer thread, to start.  Previously
a Bash script was used to do so, only to run

    settrans -c /servers/socket/1 /hurd/plfocal

This translator can also be stored in the file system, however.

If the Hurd is booted using the "--x-xattr-translator-records" option it will
use translators stored using the xattr extension for the Hurd, as
"gnu.translator" attributes.  Translators can then also be created by using
the generic `setfattr' tool, like so

    touch /servers/socket/1
    setfattr --name=gnu.translator --value='"/hurd/pflocal\0"' /servers/socket/1

Although Linux has already reserved an xattr index for the Hurd, it still
needs a patch like this

    274a93a9 gnu: linux-libre: Add support for gnu.* namespace.

one provided to support setting "gnu.translator" extended attributes.  The
Hurd needs only a trivial patch

    a3f5850b gnu: hurd: Update to upstream Hurd-reserved xattr index.

See also

    https://lists.gnu.org/archive/html/bug-hurd/2020-05/msg00016.html
    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3980bd3b406addb327d858aebd19e229ea340b9a
    https://lists.gnu.org/archive/html/bug-hurd/2016-08/msg00075.html

* gnu/bootloader/grub.scm (grub-configuration-file): Use
-x-xattr-translator-records when booting the Hurd.
* gnu/system/vm.scm (qemu-image): Include attr in native tools (for setattr).
* gnu/build/linux-boot.scm (make-hurd-device-nodes): Use it to add pflocal
translator.
* gnu/packages/hurd.scm (hurd): Remove runsystem.sh from boot proces, boot
straight into `RC".