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]

[Matthias Schiffer: rebase]

Not useful by itself except for testing; will be used for multi-domain
support.

[Matthias Schiffer: rename script, use for initial configuration]

This should not convert JSON to a Lua table and back, as this loses the
distinction between arrays and objects, but as our site.conf is defined in
Lua anyways (for now), this can be fixed in a later revision.

[Matthias Schiffer: rename to gluon-show-site, rebase]

[Matthias Schiffer: change section name and commit message]

By basing the Lua gluon.site module on gluonutil_load_site_config(), the
config load implementation needs to changed only in a single place for
multi-domain support.

Fixes #1311