Forces KRT rescan on syncer startup with multi syncer config.

Thanks to  Sergey Popovich for the patch.
Implements 'allow local as' option.

Similar to allowas-in option on other routers.
Accepts a change of OSPFv3 neighbor's IP address.

Thanks to Pierre Pfister for the patch.
Merge remote-tracking branch 'bird/master'

Conflicts:
	proto/ospf/hello.c