[B.A.T.M.A.N.] Basic Multicast Optimizations

This is the second revision of the basic multicast optimization patches. It includes one functional fix and many style fixes, thanks to Antonios feedback.

Three things were not included from these suggestions:

* The default mcast TVLV flags were left as it is not as easy to switch. (see answer/explanation to [PATCH 2/3]) * for_each_pmc_rcu() was not moved to a commen net header file, only a TODO was added. Will do that after this code might have landed in net.

Additionally the limitation of 255 multicast listeners per originator was removed (it was more a relic of the pre-TT announcement mechanism and such limitations should be taken care of by the current TT infrastructure instead).

Cheers, Linus