batman-adv: set skb priority according to content

The skb priority field may help the wireless driver to choose the right
queue (e.g. WMM queues). This should be set in batman-adv, as this
information is only available here.

This patch adds support for IPv4/IPv6 DS fields and VLAN PCP. Note that
only VLAN PCP is used if a VLAN header is present. Also initially set
TC_PRIO_CONTROL only for self-generated packets, and keep the priority
set by higher layers.

Signed-off-by: Simon Wunderlich <simon@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>