Starts a usermessage (network message).
Message index to start.
Array containing player indexes to broadcast to.
Number of players in the array.
Optional flags to set.
A handle to a bf_write bit packing structure, or INVALID_HANDLE on failure.
Invalid message name, unable to start a message, invalid client, or client not connected.
Only one message can be active at a time.
It is illegal to send any message while a non-intercept hook is in progress.