Posix: Fix no task switching issue if a task ended its main function (#184)

Posix: Fix no task switching issue if a task ended its main function (#184)

* Posix: Fix no task switching issue if a task ended

When the main function of a task exits, no task switching happened.
This is because all the remaining tasks are waiting on the condition
variable. The fix is to trigger a task switch and mark the exiting
task as "Dying" to be suspened and exited properly from the scheduler.

* Posix: Assert and stop if the Task function returned

* Posix: just assert if a task returned from its main function

Co-authored-by: alfred gedeon <alfred2g@hotmail.com>