- lets ptrace_child become void
 - adds checking for the return value of change_sig
 - moves errors info into stderr instead of stdout.

Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: NWANG Cong <wangcong@zeuux.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>