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.

configure.ac

@@ -616,9 +616,9 @@ AC_DEFINE_UNQUOTED(FALLBACK_HOSTNAME, ["$FALLBACK_HOSTNAME"], [The hostname used
 
 AC_ARG_WITH(default-hierarchy,
         AS_HELP_STRING([--with-default-hierarchy=MODE],
-                [default cgroup hierarchy, defaults to "legacy"]),
+                [default cgroup hierarchy, defaults to "hybrid"]),
         [DEFAULT_HIERARCHY="$withval"],
-        [DEFAULT_HIERARCHY="legacy"])
+        [DEFAULT_HIERARCHY="hybrid"])
 
 AS_CASE("$DEFAULT_HIERARCHY",
         [legacy], [mode=CGROUP_UNIFIED_NONE],