BGP: Add option to specify format of link-local next hop
When a BGP session is established using link-local next hop addresses,
there is no global IPv6 address for next hop. Implementations differ on
how to encode such next hop. This leads to interoperability problems.
Add the option 'link local next hop format' to specify which format to
use when encoding such next hops.
Based on a patch from Andrey V. Elsukov and Georgy Kirichenko, submitted
by Aleksandr Stepanov, thanks!