message: _nmsg_message_dup(): strip unknown fields from copied payloads
message: _nmsg_message_dup(): strip unknown fields from copied payloads

    this patch strips unknown fields from duplicated payloads.
    previously _nmsg_message_dup() was copying references to the
    heap-allocated unknown fields, which caused a double-free when
    running an nmsg_io loop in "mirror" mode.

hstern/nmsg,hstern/nmsg,hstern/nmsg,hstern/nmsg,hstern/nmsg