dnsmasq: add mx record support

Adding the following syntax support:

config mxhost
	option domain mydomain.com
	option relay svr10.ironport.com
	option pref 50

and this will generate an MX record for mydomain.com pointing at the relay with a given preference.

Redux: default the preference to 0 if absent.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@31057 3c298f89-4303-0410-b956-a3cf2f4a3e73