core: unify reset_accounting handling

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