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_test.go

@@ -26,21 +26,3 @@ func TestHasSuffix(t *testing.T) {
 		}
 	}
 }
-
-func TestToBaseName(t *testing.T) {
-	tests := []struct {
-		name string
-		want string
-	}{
-		{"foo", "foo"},
-		{"foo.com", "foo"},
-		{"foo.example.com.beta.tailscale.net", "foo"},
-		{"computer-a.test.gmail.com.beta.tailscale.net", "computer-a"},
-	}
-	for _, tt := range tests {
-		got := ToBaseName(tt.name)
-		if got != tt.want {
-			t.Errorf("ToBaseName(%q) = %q; want %q", tt.name, got, tt.want)
-		}
-	}
-}