Differentiate between PERIODIC and SPORADIC tasks.

Tasks can now be PERIODIC or SPORADIC.

PERIODIC tasks do not have their job number incremented
when they wake up and are tardy. PERIODIC jobs must
end with a call to sys_complete_job() to set up their next
release. (Not currently supported by pfair.)

SPORADIC tasks _do_ have their job number incremented when
they wake up and are tardy. SPORADIC is the default task
behavior, carrying forward Litmus's current behavior.