Add add-path support in MRT dumps (RFC8050).

Add add-path support in MRT dumps (RFC8050).

This works for table-v2 formats and also for the all and update dumps.
For the latter the code needs to inspect the UPDATE message to guess if
add-path is used in that message or not.  Add-path can be enabled per
AFI and alters the NLRI encoding without any way to detect.
For table dumps we decided to dump RIB_GENERIC entires the same way as
gobgp and not use the encoding from the RFC. The RFC specifies that
RIB_GENERIC (non-IPv4/v6) entries in a way that is not only different
but also against the normal codeflow. Only gobgp seems to implement
RIB_GENERIC encoding (neither bird nor frr/quagga do) and so it makes
sense to follow their way.

OK benno@