dccp: Cubic congestion control algorithm as new CCID-249

This patch implements the congestion control algorithm Cubic as a new
CCID for DCCP protocol. This CCID makes use of the cubic function to set
the congestion window to be used in a given time.
Moreover, we use Ack Vectors as the mechanism adopted for confirmation
of received packets. Due to that, the implementation is largely based on
the implementations of Tcp Cubic and CCID-2.

Since the RFC4340 specifies the experimental CCIDs must use ids greater
than 248, we choose 249 as the id to be used, at least for now. However,
we placed the implementation code in a pair of files named ccid5.{h,c}
(since this would be the next available CCID).

Signed-off-by: Ivo Calado <ivo.calado@ee.ufcg.edu.br>
Signed-off-by: Leandro Sales <leandroal@gmail.com>