introduce "struct wait_opts" to simplify do_wait() patches

Introduce "struct wait_opts" which holds the parameters for misc helpers
in do_wait() pathes.

This adds 13 lines to kernel/exit.c, but saves 256 bytes from .o and imho
makes the code much more readable.

This patch temporary uglifies rusage/siginfo code a little bit, will be
addressed by further cleanups.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>