Only perform DNSLink lookups on fully qualified domain names (FQDN)

This change halves the number of DNS queries requires to lookup DNSLink
information for "example.com" by forcing the use of a FQDN.

* example.com
* example.com.local (removed)
* _dnslink.example.com
* _dnslink.example.com.local (removed)

Where .local is the local system's organization/domain name.

namesys/dns.go

@@ -45,6 +45,7 @@ type lookupRes struct {
 // TXT records for a given domain name should contain a b58
 // encoded multihash.
 func (r *DNSResolver) resolveOnceAsync(ctx context.Context, name string, options opts.ResolveOpts) <-chan onceResult {
+	var fqdn string
 	out := make(chan onceResult, 1)
 	segments := strings.SplitN(name, "/", 2)
 	domain := segments[0]
@@ -56,11 +57,17 @@ func (r *DNSResolver) resolveOnceAsync(ctx context.Context, name string, options
 	}
 	log.Debugf("DNSResolver resolving %s", domain)
 
+	if (strings.HasSuffix(domain, ".")) {
+		fqdn = domain
+	} else {
+		fqdn = domain+"."
+	}
+
 	rootChan := make(chan lookupRes, 1)
-	go workDomain(r, domain, rootChan)
+	go workDomain(r, fqdn, rootChan)
 
 	subChan := make(chan lookupRes, 1)
-	go workDomain(r, "_dnslink."+domain, subChan)
+	go workDomain(r, "_dnslink."+fqdn, subChan)
 
 	appendPath := func(p path.Path) (path.Path, error) {
 		if len(segments) > 1 {