Merge pull request #969 from N0YB/RELENG_2_1

XHTML Compliance
Properly compile the query to insert the values. Pointy-hat: myself. While here respect the redirurl when passed to portal_allow and use proper function to do redirection.
Properly detect when there are issues with communicating with syncip and to use the local DB for this. Otherwise detect if the remote says the voucher is not valid say its not valid.
Make is_linklocal case-insensitive and fix #3433
Fix #3483 only use IPv4 DNS servers in DHCP v4 conf

Version for 2.1 branch
Merge pull request #971 from phil-davis/RELENG_2_1

Fix #3483 only use IPv4 DNS servers in DHCP v4 conf
XHTML Compliance

Firewall - NAT - Port Forward