dnsmasq: rework init procedure

	- cache udhcp check results to speed up subsequent reloads
	- enable procd file tracking for /var/etc/dnsmasq.conf to only reload service if needed
	- implement reload action to only restart dnsmasq if /var/etc/dnsmasq.conf actually changed
	- launch dnsmasq from interface hotplug to avoid race conditions with network bringup

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39152 3c298f89-4303-0410-b956-a3cf2f4a3e73