This fixes a circular dependency between (gnu system) and (gnu system
linux-initrd), where the latter could end up being compiled before
'file-system-type' was defined as a macro.
* gnu/system.scm (<file-system>, %fuse-control-file-system,
%binary-format-file-system): Move to...
* gnu/system/file-systems.scm: ... here. New file.
* build-aux/hydra/demo-os.scm, gnu/system/linux-initrd.scm,
gnu/system/vm.scm: Use it.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.
