dccp: Increase the scope of variable-length htonl/ntohl functions

dccp: Increase the scope of variable-length htonl/ntohl functions

This extends the scope of two available functions, encode|decode_value_var,
to work up to 6 (8) bytes, to match maximum requirements in the RFC.

These functions are going to be used both by general option processing and
feature negotiation code, hence declarations have been put into feat.h.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>