PSN-EDF: fix admission of suspended task

The PSN-EDF plugin used to crash when trying to admit a suspended
task. This patch fixes the task_new() handler to tolerate this case.
P-FP: fix admission of suspended task

This commit fixes admission of suspended tasks under the P-FP plugin
analogously to commit 78ae3306ff8b63a4592044aa28c2f2cbc1d36b20.
GSN-EDF: fix admission of suspended tasks

In the task_new() handler, don't process a job arrival event if the
task is actually suspended. This fixes the admission of suspended
tasks.
C-EDF: fix admission of suspended tasks

This patch fixes admission of suspended tasks under the P-FP plugin
analogously to commit 3088111567f334cccff1123f9bd97f2ba52af944.
PFAIR: fix admission of suspended tasks

The PFAIR would not properly schedule tasks that were admitted while
being suspended. This patch changes the task_new() handler to set the
"needs to be requeued" flag on suspended tasks, which is required to
ensure that they are processed correctly by the wake_up() handler.
Clean up 'time_slice' and 'completed' usage

The 'time_slice' parameter has not been relevant in a couple of years.
The 'completed' flag was misused in a few places. This clean-up patch
removes these outdated remnants.