Rework thread life changes and remove Giant lock

[Sebastian Huber]

This patch series reworks the thread life changes (cancel, exit, join, etc.)
and removes the Giant lock.  The following tickes are addressed:

https://devel.rtems.org/ticket/2280
https://devel.rtems.org/ticket/2402
https://devel.rtems.org/ticket/2555
https://devel.rtems.org/ticket/2626
https://devel.rtems.org/ticket/2714
https://devel.rtems.org/ticket/2717