libpcap: add support for B.A.T.M.A.N. Advanced

This adds support for the layer 2 mesh routing protocol
B.A.T.M.A.N. Advanced. "batadv" can be used to filter on batman-adv
packets. It also allows later filters to look at frames inside the
tunnel when both "version" and "type" are specified.

Documentation for the batman-adv protocol can be found at the following
locations:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/networking/batman-adv.rst
https://www.open-mesh.org/

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This is a backport of the following upstream pull request:

the-tcpdump-group/libpcap#980
-> "Add support for B.A.T.M.A.N. Advanced coolsnowwolf#980"

Signed-off-by: Linus Lüssing <[email protected]>