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.
If the caller doesn't specify ancillary data, add credentials to the
outgoing packet.
This enables us to satisfy the requirement
(https://man7.org/linux/man-pages/man7/unix.7.html) that a socket with
the SO_PASSCRED option enabled can get the credentials of its peer in
every message it receives.
FIXME: I'm not sure if this is the right way to satisfy that
requirement. A possible alternative would be to arrange for a socket
to be notified when its peer enables SO_PASSCRED.
