Datagram: add add_server_header and add_control_header and constructors that call them
MessageDirector: Use new Datagram constructors for control messages
Merge branch 'master' of github.com:OpenOTP/OpenOTP
Datagram: Use CONTROL_MESSAGE macro.
Datagram: Add constructor/add_server_header for a set of channels.
Datagram: Add method for appending a sub-datagram.
Datagram: Remove unneeded member.
Datagram: add_datagram takes a const
Merge pull request #30 from OpenOTP/cfsworks-dgcleanup

Clean up datagram structure