build-sys: default to hybrid cgroup hierarchy

It is expected that general-purpose distributions might want to override this.
This commit is made separate from grandparent to make it easy to revert if
needed.

v2:
- use hybrid as the default
  (We tested that the default of unified seems boot correctly everywhere and behave
   correctly in general, but it is incompatible with docker/lxc and probably some
   other tools, so for now we default to hybrid. The new "hybrid" mode should work
   be OK for those tools.)

Fixes #4669.

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