config: Set the list separator to be comma

The code did not set the list separator so semicolon was used by
default. The documentation in config-format.txt says the list
items are separated by comma.
resolver: Use fallback nameservers when none are otherwise supplied

Fallback nameservers are added to the nameserver list with their
interface index set to -1. When a nameserver entry is removed, check
if the fallback ones should be used instead. When nameserver entries
have been added and flushed and at least one of them has an
interface index, remove all fallback nameservers.

Fixes CM-652
service: Add fallback nameservers to the Nameservers property
main.conf: Update FallbackNameservers description