lib: add link-state prefixes

lib: add link-state prefixes

Add to the library the link-state type of prefixes.

Link-state prefixes contain much more data than the current prefixes and
they only make sense for BGP Link-State. Storing all the data in "struct
prefix" is not relevant because it would increase the memory usage of
all daemons. Instead a pointer to a structure that contains all the
information is used. Printing link-state prefixes can be delegated to a
hook function.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>