dnsmasq: Multiple dnsmasq instances support

Adds support in uci for configuring multiple dnsmasq instances via
multiple dnsmasq sections.
The uci sections host, boot, mac, tag, vendorclass, userclass,
circuitid, ... will refer to a dnsmasq instance via the instance
parameter defined in the section; if the instance parameter is
not specified backwards compatibility is preserved.

Start/Stopping a dnsmasq instance can be achieved by passing the
dnsmasq instance name as argument to start/stop via the init script.

Multiple dnsmasq instances is usefull in scenarios where you want to
bind a dnsmasq instance to an interface in order to isolate networks.

This patch is a rework of a multiple dnsmasq instance patch by Daniel Dickinson

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>