dnsmasq: add specific interface procd triggers

dnsmasq: add specific interface procd triggers

Right now interface.update events are sent out by netifd upon interface state,
route, address (lifetime), prefix lifetime changes.
Dnsmasq is only interested in interface state changes and currently adds an
interface trigger for all the "interface.*" events.
In combination with commit 23bba9cb330, which triggers a SIGHUP signal to dnsmasq,
IPv6 address/prefix lifetime changes on the wan will trigger dnsmasq reloads which
can become frequent in case of shorter lifetimes.

To avoid frequent dnsmasq reload, this patch adds specific interface triggers.
During dnsmasq init it loops dhcp uci section; if the value of the ignore option
is set to 0, then the corresponding interface trigger is not installed.
Otherwise, if the ignore option value is 1, then procd_add_interface_trigger is
called which adds the interface trigger.

Signed-off-by: hux <xinxing.huchn@gmail.com>