dccp: Integration of dynamic feature activation - part 2 (server side)

This patch integrates the activation of features at the end of negotiation
into the server-side code.

Note regarding the removal of 'const':
--------------------------------------
 The 'const' attribute has been removed from 'dreq' since dccp_activate_values()
 needs to operate on dreq's feature list. Part of the activation is to remove
 those options from the list that have already been confirmed, hence it is not
 purely read-only.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>