Implemnt the RFC8050 in the MRT parser. With this MRT dumps containing

Implemnt the RFC8050 in the MRT parser. With this MRT dumps containing
add-path information will be dumped properly.

There is one bit where this implementation is not in line with the RFC.
The encoding of DUMP_V2_RIB_GENERIC_ADDPATH for non IPv4 or IPv6 prefixes
is implemented the way gobgp did it (which seems to be the only other BGP
implementation that supports generic encoding). The RFC puts the path_id
in the NLRI for generic encoding instead of adding it to RIB entry struct
like in the IPv4 and IPv6 encoding. This is an open discussion point with
the RFC author.

OK benno@