batman-adv: move smallest_signed_int(), seq_before() and seq_after() into main.h

smallest_signed_int(), seq_before() and seq_after() are very useful
functions that help to handle comparisons between sequence numbers.
However they were only defined in vis.c. With this patch every
batman-adv function will be able to use them.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>