dnsmasq: switch to /lib/functions/network.sh

This commit changes the dnsmasq init script to use the interface
status exposed by netifd. The old references to scan_interfaces()
and (indirect) accesses to uci state variables are removed and
replaced with corresponding network_*() calls.

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

SVN-Revision: 39101