DNS_UPSTREAM_PLAIN_ADDRESSES option

- New CSV format with port, for example `ip1:port1,ip2:port2`
- retrocompatibility with `DNS_ADDRESS`. If set, force upstream type to plain and empty user-picked providers. 127.0.0.1 is now ignored since it's always set to this value internally.
- requires `DNS_UPSTREAM_TYPE=plain` must be set to use `DNS_UPSTREAM_PLAIN_ADDRESSES` (unless using retro `DNS_ADDRESS`)
- Warning log on using private upstream resolvers updated

internal/configuration/settings/settings_test.go

@@ -38,7 +38,6 @@ func Test_Settings_String(t *testing.T) {
 |       ├── Run OpenVPN as: root
 |       └── Verbosity level: 1
 ├── DNS settings:
-|   ├── DNS server address to use: 127.0.0.1
 |   ├── Upstream resolver type: dot
 |   ├── Upstream resolvers:
 |   |   └── Cloudflare