kthreads: kill CLONE_KERNEL, change kernel_thread(kernel_init) to avoid CLONE_SIGHAND

1. Remove CLONE_KERNEL, it has no users and it is dangerous.

   The (old) comment says "List of flags we want to share for kernel
   threads" but this is not true, we do not want to share ->sighand by
   default. This flag can only be used if the caller is sure that both
   parent/child will never play with signals (say, allow_signal/etc).

2. Change rest_init() to clone kernel_init() without CLONE_SIGHAND.

   In this case CLONE_SIGHAND does not really hurt, and it looks like
   optimization because copy_sighand() can avoid kmem_cache_alloc().

   But in fact this only adds the minor pessimization. kernel_init()
   is going to exec the init process, and de_thread() will need to
   unshare ->sighand and do kmem_cache_alloc(sighand_cachep) anyway,
   but it needs to do more work and take tasklist_lock and siglock.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>