batman-adv: enable fast client detection using unicast_4addr packets

The "early client detection mechanism" can be extended to find new clients by
means of unicast_4addr packets.

The unicast_4addr packet contains as well as the broadcast packet (which is
currently used in this mechanism) the address of the originating node and can
therefore be used to install new entries in the Global Translation Table

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>