sched: improve the rt-throttling interface

The EDF-throttling interface as it is is quite unusable.  Add support
for '-1' as a special value for the rt_task_runtime_us cgroup attribute,
assigning to the tasks of the group the same period as their parent group
and all the free runtime available within that group.

Signed-off-by: Fabio Checconi <fchecconi@gmail.com>
Signed-off-by: Juri Lelli <juri.lelli@gmail.com>