Harmonize `name server` and `nameserver`

[jsoref]

https://github.com/search?q=repo%3Azonemaster%2Fzonemaster-engine+%2F+name+%3Fserver+%5B%5E%3D%5D%2F+NOT+language%3A%22Gettext+Catalog%22+language%3APerl+&type=code

There are probably times where it makes sense to use name server instead of nameserver, however,...

https://github.com/search?q=repo%3Azonemaster%2Fzonemaster-engine+%2F%5B%22+%5Dname+%3Fserver+%5B%7B%5D%2F+NOT+language%3A%22Gettext+Catalog%22+language%3APerl+&type=code

https://github.com/zonemaster/zonemaster-engine/blob/17688f7995e16dd9f5a4bec2a1437fc7804b31f0/lib/Zonemaster/Engine/Translator.pm#L84 https://github.com/zonemaster/zonemaster-engine/blob/17688f7995e16dd9f5a4bec2a1437fc7804b31f0/lib/Zonemaster/Engine/Translator.pm#L88 https://github.com/zonemaster/zonemaster-engine/blob/17688f7995e16dd9f5a4bec2a1437fc7804b31f0/lib/Zonemaster/Engine/Test/Delegation.pm#L278 https://github.com/zonemaster/zonemaster-engine/blob/17688f7995e16dd9f5a4bec2a1437fc7804b31f0/lib/Zonemaster/Engine/Test/Nameserver.pm#L353