system: Add '%devtmpfs-file-system' for udev, and '%base-file-systems'.
Suggested by Adam Pribyl <pribyl@lowlevel.cz>.
* gnu/services/base.scm (udev-service)[requirement]: Add
'file-system-/dev'.
* gnu/system/file-systems.scm (%devtmpfs-file-system,
%base-file-systems): New variables.
* gnu/system/install.scm (installation-services)[file-systems]: Use
%base-file-systems.
* build-aux/hydra/demo-os.scm (file-systems): Likewise.
* doc/guix.texi (System Installation): Show %BASE-FILE-SYSTEMS in the
example.
(Using the Configuration System): Likewise.
(File Systems): Document %base-file-systems, %devtmpfs-file-system,
%binary-format-file-system, and %fuse-control-file-system.
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