In multidomain setups, VXLAN is enabled by default, but can be disabled in
domain configs using the mesh/vxlan option. In single domain setups, the
mesh/vxlan option is mandatory.

The UCI option for legacy mode is removed.

Fixes #1364

gluon-core, gluon-l3roamd: introduce script gluon-list-mesh-interfaces that lists all currently active mesh interfaces

dnsmasq's caching is severly broken and does not handle all answer records
equally. In particular, its cached answers are missing DNSKEY and DS
records, breaking DNSSEC validation on clients.

Remove the cache for now. It may return if dnsmasq is fixed or we switch to
a different resolver.

The script cleaned up an old setting that was removed in 2014.

net.ipv6.conf.br-client.forwarding is moved from gluon-client-bridge to
gluon-mesh-batman-adv, as the setting is not useful with non-bridged
protocols.

Our VXLAN setup was changed to accept VXLAN packets without checksum almost
2 months ago, so we can disable sending the checksums now as well. Slightly
improves performance.

exec() is moved to gluon.util.

* change type of next_node.name
* create domain entry for each entry and add to dnsmasq configuration

[Matthias Schiffer: reword documentation]

Both gluon.sysconfig and libgluonutil already remove the trailing newline
if it exists. It's nicer to avoid files without a trailing newline, e.g.
for printing the file contents in a terminal.

Patch originally authored by @oleeander.

Fixes #424

Allows to remove some redundant UCI lookups.

Fixes #1322

The prefix is not used, and requesting it leads to odhcp6c log spam with
certain DHCPv6 servers.

Based-on-patch-by: lemoer <git@irrelefant.net>

This does not do anything yet, as this_domain() is not implemented yet.

Based-on-patch-by: lemoer <git@irrelefant.net>

Also disabling TX checksums and not only allowing incoming packets without
checksum will provide another small speedup. As doing so would break wired
meshing with VXLAN-enabled nodes that require non-zero checksums, we will
wait a few days before this step.

Evaluating these rules before all the ICMPv6 rules improves wired mesh
throughput measurably.

On most devices, there is only a single LAN interface connected to all LAN
ports, so no bridge is necessary.

In addition to significant internal differences in check_site_lib.lua (in
particular unifying error handling to a single place for the upcoming
multi-domain support), this changes the way fields are addressed in site
check scripts: rather than providing a string like 'next_node.ip6', the
path is passed as an array {'next_node', 'ip6'}.

Other changes in site check scripts:
* need_array and need_table now pass the full path to the sub fields to the
subcheck instead of the key and value
* Any check referring to a field inside a table implies that all higher
levels must be tables if they exist: a check for {'next_node', 'ip6'} adds
an implicit (optional) check for {'next_node'}, which allows to remove many
explicit checks for such tables

[Matthias schiffer: rebase, add a few more restrictions]