dnsmasq: allow using dnsmasq as the sole resolver

dnsmasq: allow using dnsmasq as the sole resolver

Currently it seems impossible to configure /etc/config/dhcp to achieve
the following use case

 - run dnsmasq with no-resolv
 - re-generate /etc/resolv.conf with "nameserver 127.0.0.1"

Before this change, we have to set resolvfile to /tmp/resolv.conf.auto
to achive the 2nd effect above, but setting resolvfile requires noresolv
being false.

A new boolean option "localuse" is added to indicate that we intend to
use dnsmasq as the local dns resolver.  It's false by default and to
align with old behaviour it will be true automatically if resolvfile is
set to /tmp/resolv.conf.auto

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Acked-by: Hans Dedecker <dedeckeh@gmail.com>