cgroup v2: Don't require CPU controller for CPU accounting in 4.15+

systemd only uses functions that are as of Linux 4.15+ provided
externally to the CPU controller (currently usage_usec), so if we have a
new enough kernel, we don't need to set CGROUP_MASK_CPU for
CPUAccounting=true as the CPU controller does not need to necessarily be
enabled in this case.

Part of this patch is modelled on an earlier patch by Ryutaroh Matsumoto
(see PR #9665).

src/basic/cgroup-util.h

@@ -69,6 +69,9 @@ static inline CGroupMask CGROUP_MASK_EXTEND_JOINED(CGroupMask mask) {
         return mask;
 }
 
+CGroupMask get_cpu_accounting_mask(void);
+bool cpu_accounting_is_cheap(void);
+
 /* Special values for all weight knobs on unified hierarchy */
 #define CGROUP_WEIGHT_INVALID ((uint64_t) -1)
 #define CGROUP_WEIGHT_MIN UINT64_C(1)