core: unify reset_accounting handling

Since the introduction of CGroupRuntime, there's no need
to call *_reset_accounting in unit_new(), hence make those
static. While at it, refrain from hardcoding default values
in cgroup_runtime_new(), but call the corresponding funcs.

This also corrects the default value of io_accounting_base.

Fixes #33482

src/core/unit.c

@@ -129,9 +129,6 @@ Unit* unit_new(Manager *m, size_t size) {
                 .burst = 16
         };
 
-        unit_reset_memory_accounting_last(u);
-        unit_reset_io_accounting_last(u);
-
         return u;
 }