nsrep: allow inclusion of good nameservers if there's space in selection

The current algorithm doesn't work if there's 3 unknown nameservers in the address selection set, and the 4th is known working with higher score than unknown.

@vavrusam: I noticed you pushed this simple commit, so I picked it. It seems good, and I can't see anything noticeable in large repeated iterative respdiff tests (this code-path won't be reached when forwarding).

added 257 commits

• 036d2ade...019ce4fb - 256 commits from branch master
• 96b4d0a6 - nsrep: allow inclusion of good nameservers if there's space in selection

Compare with previous version

added 7 commits

• 96b4d0a6...37510f73 - 6 commits from branch master
• f84df9dd - nsrep: allow inclusion of good nameservers if there's space in selection

Compare with previous version

added 9 commits

• f84df9dd...768ecc0d - 8 commits from branch master
• 196ebd4f - nsrep: allow inclusion of good nameservers if there's space in selection

Compare with previous version

merged

mentioned in commit a9724192