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.
(A knob could also be added to the GUI to let users specify the maximum cache age themselves and store it in config.xml - someone can do that also if they care, and then the config.xml can be retrieved by _maxCacheAge() )