Check dynamic DNS cache age by provider

Some forum threads have been mentioning that "noip" requires more frequent updates than every 25 days - e.g. http://forum.pfsense.org/index.php/topic,59231.0.html
This change allows the maximum cache age (in days) to be easily specified by DNS provider in the _maxCacheAge() function.
"noip" is set to 7 days.
All other providers remain at the previous value of 25 days.