Add new port net/nmsg

PR: 190601
Submitted by: Don Lewis (truckman@)

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The NMSG format is an efficient encoding of typed, structured data into
payloads which are packed into containers which can be transmitted over
the network or stored to disk. libnmsg is the reference implementation
of this format and provides an extensible interface for creating and
parsing messages in NMSG format. The NMSG format relies on Google
Protocol Buffers to encode the payload header. Individual NMSG payloads
are distinguished by assigned vendor ID and message type values and
libnmsg provides a modular interface for registering handlers for
specific message types. libnmsg makes it easy to build new message
types using the protobuf-c compiler.