Remove special handling of certain upstream DNS server

[yubiuser]

What does this PR aim to accomplish?:

Simplifies the upstream DNS server count/naming. We had special "names" for some upstream servers (Cloudflared, gateway) but the choice seems a bit arbitrary. If we had more than 8 upstream DNS server, we also would indicated this.

This PR allows only 2 cases now:

1. exactly 1 server
2. more than 1 server

P.S. Removes now unused variable $pi_gateway

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