Add rt_task_next_period to user-space

next_period was useful to get the time left until next period of the
task, but it was only in kernel space.  This commit changes that to
rt_task_next_period, and adds a NEXT_PERIOD syscall that lets that
functionality be used in user-space too.

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>