unbound: add odhcpd specific scripts to link DHCP-DNS

unbound: add odhcpd specific scripts to link DHCP-DNS

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 <redacted>