We now create bat0 and primary0 independently of the lower mesh interfaces,
making the whole setup a lot more robust. In particular:
- we can't accidentially destroy primary0 because of concurrent setup and
teardown runs of different interfaces
- bat0 will always exist, even when no mesh interfaces are up (e.g. no link
on wired mesh)
- interfaces going down and up again will never tear down the whole of
batman-adv
- we can enable and disable bat0 independently of the lower interface
states
