The previous setsockopt interface, which passed socket options via struct 
dccp_so_feat, is complicated/difficult to use. Continuing to support it leads to
ugly code since the old approach did not distinguish between NN and SP values.

This patch removes the old setsockopt interface and replaces it with two new
functions to register NN/SP values for feature negotiation. These are 
essentially wrappers around the internal __feat_register functions, with 
checking added to avoid
 * wrong usage (type);
 * changing values while the connection is in progress.
Signed-off-by: NGerrit Renker <gerrit@erg.abdn.ac.uk>