ndnd: Correction of TLV/NDNb flag switching

The correction applies to UDP faces: after receiving a datagram packet,
now the flag is set on a correct face---face to the neighbor, not the
generic face used to receive and dispatch UDP datagrams.

Change-Id: I04727585ba925307c707482a3ba9247e8b49854e
tlv-hack: Another correction for UDP faces

When sending packets over UDP faces, NDNd bundles them with additional
"SequenceNumber" message. This commit introduces support for this
SequenceNumber message (to and from conversions), and enables conversion
of multiple NDNb-encoded messages within one ndnb_to_tlv function call.

Change-Id: I683abf7af54b768a0596bb6a7ed64b52ffcfd907