task/pthread_cancelpt: Move cancel point handling to libc, data to TLS

This moves task / thread cancel point logic from the NuttX kernel into
libc, while the data needed by the cancel point logic is moved to TLS.

The change is an enabler to move user-space APIs to libc as well, for
a coherent user/kernel separation.

fs/procfs/fs_procfsproc.c

@@ -578,9 +578,7 @@ static ssize_t proc_status(FAR struct proc_file_s *procfile,
   /* Show task flags */
 
   linesize = procfs_snprintf(procfile->line, STATUS_LINELEN,
-                          "%-12s%c%c%c\n", "Flags:",
-                          tcb->flags & TCB_FLAG_NONCANCELABLE ? 'N' : '-',
-                          tcb->flags & TCB_FLAG_CANCEL_PENDING ? 'P' : '-',
+                          "%-12s%c\n", "Flags:",
                           tcb->flags & TCB_FLAG_EXIT_PROCESSING ? 'P' : '-');
 
   copysize   = procfs_memcpy(procfile->line, linesize, buffer, remaining,