Use a struct sockaddr_in pr struct sockaddr_in6 as the option value
for the IPPROTO_SCTP level socket options SCTP_BINDX_ADD_ADDR and
SCTP_BINDX_REM_ADDR. These socket option are intended for internal
use only to implement sctp_bindx().
This is one user of struct sctp_getaddresses less.
struct sctp_getaddresses is strange and will be changed shortly.