tailcfg: update node display name fields and methods (#1207)

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

Consolidates the node display name logic from each of the clients into
tailcfg.Node. UI clients can use these names directly, rather than computing
them independently.

util/dnsname/dnsname.go

@@ -17,11 +17,3 @@ func HasSuffix(name, suffix string) bool {
 	nameBase := strings.TrimSuffix(name, suffix)
 	return len(nameBase) < len(name) && strings.HasSuffix(nameBase, ".")
 }
-
-// ToBaseName removes the domain ending from a DNS name of a node.
-func ToBaseName(name string) string {
-	if i := strings.Index(name, "."); i != -1 {
-		return name[:i]
-	}
-	return name
-}