[FS#597] Unchecking "Use DNS servers advertised by peer" has no effect for IPv6

[LEDE Bugs]

For full information about what has changed in this task, visit the URL below and click the History tab.

FS#597 - Unchecking "Use DNS servers advertised by peer" has no effect for IPv6
User who did this: Jo-Philipp Wich (jow-)
Percent Complete: 0% -> 50%

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=597