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

For simplicity, we don't use different MTUs for compat 14 and 15 anymore,
there's no harm in using 1532 for batman-adv-legacy as well.

In addition, transitive and fixed_mtu now default to 0 instead of an empty string.

gluon-core: gluon.util: pass UCI cursor as argument to iterate_radios() instead of using local context

batman-adv 2017.0 doesn't need (or support) no_rebroadcast anymore. Don't
fail when the file doesn't exist.

Completely untested for now.

Closes #835

VPN daemons were always disabled

The TL-WR940N doesn't have detachable antennas. Make v1-v3 match v4.

Fixes meshing over 802.11s.

Doing so caused broken validations, as different validators were affecting
each other.

Based-on-patch-by: Cyrus Fox <cyrus@lambdacore.de>

Based-on-patch-by: Cyrus Fox <cyrus@lambdacore.de>

The fastd_mesh_vpn site.conf section is renamed to mesh_vpn.fastd.