add support for cgroup runtime shared: allow the cgroup to share "not used" runtime.
in the classic cgroup implementention, the cpu time of each cgroup must not be greater than its rt_runtime, even the cpu is idle. we change this behavior. the root cgroup's cpu time is still less than its rt_runtime but all non-root cgroups' cpu time can be greater its rt_runtime. we implement by sharing the "not used" rt_runtime among all non-root cgroups.
the "not used" runtime is composed by two parts:
1.the fixxed part, runtime of the root cgroup minus the sum of runtimes of all leaf cgroups;
2.the dynamic part, the sum of runtime for each leaf cgroups minus the sum of time that each leaf cgroup really used.