Ability to use --all-nameservers with IPv4 and IPv6 at the same time

[pspacek]

Version: commit 92eb2f4

Describe the feature requested
--iterative --all-nameservers CLI option does not actually query all servers - by default it queries only IPv4 servers.

Both families cannot be requested at the same time:

FATA[0000] Cannot specify both --prefer-ipv4-iteration and --prefer-ipv6-iteration

Additional context
Use-case: Detecting inconsistencies in answers between IPv4 and IPv6 name servers.

Solution
Personally I would think both address families should be used by default and knobs --prefer-ipv4-iteration and --prefer-ipv6-iteration can be used to restrict queries to a given address family.

[pspacek]

FTR this quick hack does not cut it:

diff --git a/src/modules/nslookup/ns_lookup.go b/src/modules/nslookup/ns_lookup.go
index 8c77d2d..e6bbac8 100644
--- a/src/modules/nslookup/ns_lookup.go
+++ b/src/modules/nslookup/ns_lookup.go
@@ -43,8 +43,9 @@ func (nsMod *NSLookupModule) CLIInit(gc *cli.CLIConf, resolverConf *zdns.Resolve
                return errors.New("NSLOOKUP module does not support --all-nameservers")
        }
        if !nsMod.IPv4Lookup && !nsMod.IPv6Lookup {
-               log.Debug("NSModule: neither --ipv4-lookup nor --ipv6-lookup specified, will only request A records for each NS server")
+               log.Debug("NSModule: neither --ipv4-lookup nor --ipv6-lookup specified, will request A+AAAA records for each NS server")
                nsMod.IPv4Lookup = true
+               nsMod.IPv6Lookup = true
        }
        err := nsMod.BasicLookupModule.CLIInit(gc, resolverConf)
        if err != nil {

The resolver attribute is empty for IPv6 endpoints, but it seems number of results is correct.