cmd/tailscale: change formatting of "tailscale status"

* show DNS name over hostname, removing domain's common MagicDNS suffix.
  only show hostname if there's no DNS name.
  but still show shared devices' MagicDNS FQDN.

* remove nerdy low-level details by default: endpoints, DERP relay,
  public key.  They're available in JSON mode still for those who need
  them.

* only show endpoint or DERP relay when it's active with the goal of
  making debugging easier. (so it's easier for users to understand
  what's happening) The asterisks are gone.

* remove Tx/Rx numbers by default for idle peers; only show them when
  there's traffic.

* include peers' owner login names

* add CLI option to not show peers (matching --self=true, --peers= also
  defaults to true)

* sort by DNS/host name, not public key

* reorder columns

util/dnsname/dnsname.go

@@ -0,0 +1,19 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package dnsname contains string functions for working with DNS names.
+package dnsname
+
+import "strings"
+
+// HasSuffix reports whether the provided DNS name ends with the
+// component(s) in suffix, ignoring any trailing dots.
+//
+// If suffix is the empty string, HasSuffix always reports false.
+func HasSuffix(name, suffix string) bool {
+	name = strings.TrimSuffix(name, ".")
+	suffix = strings.TrimSuffix(suffix, ".")
+	nameBase := strings.TrimSuffix(name, suffix)
+	return len(nameBase) < len(name) && strings.HasSuffix(nameBase, ".")
+}

util/dnsname/dnsname_test.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package dnsname
+
+import "testing"
+
+func TestHasSuffix(t *testing.T) {
+	tests := []struct {
+		name, suffix string
+		want         bool
+	}{
+		{"foo.com", "com", true},
+		{"foo.com.", "com", true},
+		{"foo.com.", "com.", true},
+
+		{"", "", false},
+		{"foo.com.", "", false},
+		{"foo.com.", "o.com", false},
+	}
+	for _, tt := range tests {
+		got := HasSuffix(tt.name, tt.suffix)
+		if got != tt.want {
+			t.Errorf("HasSuffix(%q, %q) = %v; want %v", tt.name, tt.suffix, got, tt.want)
+		}
+	}
+}