bgpd - graceful restart capability


we mostly want this for the End-of-RIB marker.
After router is done with a bulk transfer, it sends an empty UPDATE to indicate the fact.

could use that and router refresh for filter updates without extra copies of the received routes.
soft-reconfig auto

also useful for interaction with carp, only become master after the table is there.
disabled right now, bizarre problem in capabilities negotiation with juniper, they quote incorrectly in the NOTIFICATION when graceful restart is disabled on their side. beeing worked on with them.