batman-adv: split routing API data structure in subobjects

The routing API data structure contains several function
pointers that can easily be grouped together based on the
component they work with.

Split the API in subobjects in order to improve definition readability.

At the same time, remove the "bat_" prefix from the API object and
its fields names. These are batman-adv private structs and there is no
need to always prepend such prefix, which only makes function invocations
much much longer.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Reviewed-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>