Previously, create_cmsg_data and evaluate_cmsg_data required the
ancillary data to contain only a single control message, of type
SCM_CREDENTIALS. In preparation for supporting SCM_RIGHTS in the
future, allow more than one.
create_cmsg_data now iterates through the specified control messages
and allows both SCM_CREDENTIALS and SCM_RIGHTS. If no SCM_CREDENTIALS
message is present, it creates one. This was previously done in
sendmsg.
evaluate_cmsg_data also iterates through the received control messages
and allows both SCM_CREDENTIALS and SCM_RIGHTS. Control messages of
type SCM_CREDENTIALS are discarded unless the SO_PASSCRED option has
been set.
Update tests.
