Enhanced validation of general DNS servers and gateways

Same as for the code just provided on master. I also included the changes from "strip excess whitespace from ntp field before processing" https://github.com/pfsense/pfsense/commit/8adb814b30340114407ffc7a0df9e723f7d19d80 which had also not been applied back to 2.1 branch, because they seem like "a good thing" and will not hurt anyone. IMHO might as well have as many validation helps and checks as possible in the point releases.