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>

SVN-Revision: 31057