The MRT protocol

The new MRT protocol is responsible for periodic RIB table dumps in the
MRT format (RFC 6396). Also the existing code for BGP4MP MRT dumps is
refactored and splitted between BGP to MRT protocols, will be more
integrated into MRT in the future.

Example:

protocol mrt {
	table "*";
	filename "%N_%F_%T.mrt";
	period 60;
}

It is partially based on the old MRT code from Pavel Tvrdik.