pid1: turn memory accounting on by default now

After discussions with @htejun it appears it's OK now to enable memory
accounting by default for all units without affecting system performance
too badly. facebook has made good experiences with deploying memory
accounting across their infrastructure.

This hence turns MemoryAccounting= from opt-in to opt-out, similar to
how TasksAccounting= is already handled. The other accounting options
remain off, their performance impact is too big still.

NEWS

@@ -1,5 +1,21 @@
 systemd System and Service Manager
 
+CHANGES WITH 238 in spe:
+
+        * The MemoryAccounting= unit property now defaults to on. After
+          discussions with the upstream control group maintainers we learnt
+          that the negative impact of cgroup memory accounting on current
+          kernels is finally relatively minimal, so that it should be safe to
+          enable this by default without affecting system performance too
+          much. Besides memory accounting only tasks accounting is turned on by
+          default, all other forms of resource accounting (CPU, IO, IP) remain
+          off for now, because it's not clear yet that their impact is small
+          enough to move from opt-in to opt-out for them, too. We recommend
+          downstreams to leave memory accounting on by default, however in some
+          situations it might be wise to revert this change of defaults, in
+          particular on very resource constrained systems or when support for
+          old kernels is a necessity.
+
 CHANGES WITH 237:
 
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