Base LEDE/OpenWrt UCI for dnsmasq provides for DNS override in
/etc/config/dhcp. It is desired to be able to use dnsmasq and
Unbound as transparently as possible. Option 'add_extra_dns'
will pull 'domain', 'mxhost', 'srvhost, and 'cname' from base.

netifd/procd have an interaction with DHCPv6/RA on WAN (FS#713).
Minor IP6 parameter updates can cause Unbound reload events every
few minutes. List option 'trigger' selects which interfaces may
cause reload. For example 'lan', 'wan' but not 'wan6'.

Squash other cosmetics.

Signed-off-by: Eric Luehrsen <ericluehrsen@hotmail.com>

Read UNBOUND_TXT_DOMAIN from main unbound configuration.
This prevents records to be added into Unbound in the default 'lan' zone.

Signed-off-by: Audric Schiltknecht <storm+github@chemicalstorm.org>

When for example 'package/net/adblock' and DNSSEC vs NTP robustness
is enabled, significant restart thrashing can occur at boot up. DHCP
lease triggers may be occuring at the same time. Unbounds DNS-DHCP
may be incomplete until new DHCP solicit events. Solve this by
leaving a passive but complete host conf file during lease trigger.

Signed-off-by: Eric Luehrsen <ericluehrsen@hotmail.com>

The UCI for Unbound already links to dnsmasq, but what
if with Unbound, we want to configure a plain dhcp server.
Most servers can call a script for lease events. That
script can then formulate DNS records and load them
with unbound-control (dependency).

The files added here work with OpenWRT/LEDE odhcpd, such
that it can be run alone. They can be used as examples
for any dhcp server. 'odhcpd.sh' is to be called by
odhcpd when a lease event occurs. 'odhcpd.awk' is called
internal to the shell script. The awk script handles
any tricky reformating that may be required.

/etc/config/dhcp
config odhcpd 'odhcpd'
  option leasetrigger '/usr/lib/unbound/odhcpd.sh'

Signed-off-by: Eric Luehrsen <ericluehrsen@hotmail.com>