time-util: make sure USEC_PER_SEC and friends are actually of type usec_t
cgroups: simplify CPUQuota= logic

Only accept cpu quota values in percentages, get rid of period
definition.

It's not clear whether the CFS period controllable per-cgroup even has a
future in the kernel, hence let's simplify all this, hardcode the period
to 100ms and only accept percentage based quota values.