Show also IPv6 nameservers in DEBUG_RESOLVER

[DL6ER]

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Show also possible IPv6 name servers when DEBUG_RESOLVER is enabled. They are stored in a somewhat auxiliary (external) structure so they were not included in FTL's debug outputs even when they were used during resolving host names. Hence, this PR is a purely cosmetic change without any effect on the functionality. Without DEBUG_RESOLVER being enabled, the changed printing routine is never called.

Before:

[2020-08-16 21:15:52.793 21996/T22008] Trying to resolve 127.0.0.2
[2020-08-16 21:15:52.793 21996/T22008] Using nameservers:
[2020-08-16 21:15:52.793 21996/T22008]  0: 127.0.0.1:53 (IPv4)
[2020-08-16 21:15:52.793 21996/T22008]  1: 0.0.0.0:0 (IPv4)
[2020-08-16 21:15:52.793 21996/T22008]  2: 0.0.0.0:0 (IPv4)

Now:

[2020-08-16 21:15:52.793 21996/T22008] Trying to resolve 127.0.0.2
[2020-08-16 21:15:52.793 21996/T22008] Using nameservers:
[2020-08-16 21:15:52.793 21996/T22008]  0: 127.0.0.1:53 (IPv4)
[2020-08-16 21:15:52.793 21996/T22008]  1: 0.0.0.0:0 (IPv4)
[2020-08-16 21:15:52.793 21996/T22008]  2: 0.0.0.0:0 (IPv4)
[2020-08-16 21:15:52.793 21996/T22008]  3: 0.0.0.0:53 (IPv4)
[2020-08-16 21:15:52.793 21996/T22008]  4: ::1:53 (IPv6)
[2020-08-16 21:15:52.793 21996/T22008]  5: fe80::2aea:17ff:44a3:160d:53 (IPv6)

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/force-update-of-ip-hostname-clientname-relationship/36585/54

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-core-web-v5-2-and-ftl-v5-3-released/40909/1