MINOR: ncbmbuf: implement add

MINOR: ncbmbuf: implement add

This patch implements add operation for ncbmbuf type.

This function is simpler than its ncbuf counterpart. Indeed, for now
only NCB_ADD_OVERWRT mode is supported. This compromise has been chosen
as ncbmbuf will be first used for QUIC CRYPTO frames handling, which
does not mandate to compare existing filled blocks during insertion.

As the previous patch, this commit must be backported prior to the fix
to come on QUIC CRYPTO frames parsing.