A pointer to a message.
The new length of the data portion of the IP_MsgBuffer, pointed to as message->msg.
0 on success, -1 on failure and errno is set:
ENOSYS - the message is either not resizable or not dynamic.
ENOMEM - the memory reallocation returned NULL.
This function assumes that the message encapsulates an IP_MsgBuffer, and resizes the data portion of that IP_MsgBuffer to datalen bytes through a call to ME_Realloc.