batman-adv: Mark best gateway in transtable_global debugfs

The transtable_global debug file can show multiple entries for a single client
when multiple gateways exist. The chosen gateway isn't marked in the list and
therefore the user cannot easily debug the situation when there is a problem
with the currently used gateway.

The best gateway is now marked with "*" and secondary gateways are marked with
"+".

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