sctp: Add socket option operation for Auto-ASCONF.

This patch allows the application to operate Auto-ASCONF on/off
behavior via setsockopt() and getsockopt().

Signed-off-by: Michio Honda <micchie@sfc.wide.ad.jp>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>