batman-adv: move bat_algo functions into a separate file

The bat_algo functionality in main.c is mostly unrelated to the rest of the
content. It still takes up a large portion of this source file (~15%, 103
lines). Moving it to a separate file makes it better visible as a main
component of the batman-adv implementation and hides it less in the other
helper functions in main.c.

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