unbound: add option for dhcp UCI domain mx srv and cname

unbound: add option for dhcp UCI domain mx srv and cname

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 <redacted>