Improve input validation for services_dhcpv6_relay
Make the same imrovements as was done for services_dhcp_relay in commit https://github.com/pfsense/pfsense/commit/ac999f3b7894bd8d5fc35419a22cc2df39416e42(cherry picked from commit 03444470be5c4c2d713873342acb04793c2aa561)
Improve input validation for services_dhcpv6_relay
Make the same imrovements as was done for services_dhcp_relay in commit https://github.com/pfsense/pfsense/commit/ac999f3b7894bd8d5fc35419a22cc2df39416e42
(cherry picked from commit 03444470be5c4c2d713873342acb04793c2aa561)