Reworked quick fqdn check

Quick check that a submitted fqdn uses valid characters and is less than 255 characters overall. Should still be rewritten to also check that each dot-separated segment is less than 63 characters.

To test, submit 3 domains through luci-commotion-dash: www.commotionwireless.net, www.commotionwireless.net:1337, and www.comm%00tionwireless.net. Only the first should be accepted.