Implement send side of RFC7911 ADD-PATH

Implement send side of RFC7911 ADD-PATH

This allows to send out more then one path per perfix to a neighbor that
supports add-path receive. OpenBGPD supports a few different modes to
select which paths to send:
  - all:	send all valid paths (the ones with a * in bgpctl output)
  - best:	send out only the single best path
  - ecmp:	send out paths that evaluate the same up and including
                the nexthop metric
  - as-wide-best: send out paths that evaluete the same up but not including
		  the nexthop metric
Currently ecmp and as-wide-best are the same. On top of this best, ecmp
and as-wide-best allow to include extra paths (e.g. best plus 2) and
for the multipath modes there is also a maximum (e.g. ecmp plus 2 max 4)

OK tb@