add uci support for "proxy-dnssec" in dnsmasq

This patch simply adds support for the "--proxy-dnssec" command in dnsmasq into the init file so it can be used with /etc/config/dhcp.

Signed-off-by: Adam Gensler <openwrt@kristenandadam.net>

git-svn-id: svn://svn.openwrt.org/openwrt@36570 3c298f89-4303-0410-b956-a3cf2f4a3e73