sctp: add SCTP_ASCONF_SUPPORTED sockopt

SCTP_ASCONF_SUPPORTED sockopt is used to set enpoint's asconf
flag. With this feature, each endpoint will have its own flag
for its future asoc's asconf_capable, instead of netns asconf
flag.

Note that when both ep's asconf_enable and auth_enable are
enabled, SCTP_CID_ASCONF and SCTP_CID_ASCONF_ACK should be
added into auth_chunk_list.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>