This commit renames all internal OS functions defined under sched/task so that they begin with the prefix. For example, nxtask_exit() vs. task_exit().

Squashed commit of the following:

    Trivial, cosmetic
    sched/, arch/, and include:  Rename task_vforkstart() as nxtask_vforkstart()
    sched/, arch/, and include:  Rename task_vforkabort() as nxtask_vforkabort()
    sched/, arch/, and include:  Rename task_vforksetup() as nxtask_vfork_setup()
    sched/:  Rename notify_cancellation() as nxnotify_cancellation()
    sched/:  Rename task_recover() to nxtask_recover()
    sched/task, sched/pthread/, Documentation/:  Rename task_argsetup() and task_terminate() to nxtask_argsetup() and nxtask_terminate(), respectively.
    sched/task:  Rename task_schedsetup() to nxtask_schedsetup()
    sched/ (plus some binfmt/, include/, and arch/):  Rename task_start() and task_starthook() to nxtask_start() and nxtask_starthook().
    arch/ and sched/:  Rename task_exit() and task_exithook() to nxtask_exit() and nxtask_exithook(), respectively.
    sched/task:  Rename all internal, static, functions to begin with the nx prefix.

arch/xtensa/src/common/xtensa_exit.c

@@ -173,7 +173,7 @@ void _exit(int status)
 
   /* Destroy the task at the head of the ready to run list. */
 
-  (void)task_exit();
+  (void)nxtask_exit();
 
   /* Now, perform the context switch to the new ready-to-run task at the
    * head of the list.