Rename then to goto in session states

[treiher]

goto should be more clearly understood as a state transition than then.

Proposed Syntax

      state Receive_Ack is
         [...]
      begin
         [...]
      transition
         goto Success
            if Ack'Valid = True and Ack.Op = DHCP::BOOTREPLY and Ack_Message_Type = DHCP::DHCPACK
         goto Failure
      exception
         goto Failure
      end Receive_Ack;

Current Syntax

      state Receive_Ack is
         [...]
      begin
         [...]
      transition
         then Success
            if Ack'Valid = True and Ack.Op = DHCP::BOOTREPLY and Ack_Message_Type = DHCP::DHCPACK
         then Failure
      exception
         then Failure
      end Receive_Ack;