ipn/{ipnlocal,localapi}: add API to toggle use of exit node

This is primarily for GUIs, so they don't need to remember the most
recently used exit node themselves.

This adds some CLI commands, but they're disabled and behind the WIP
envknob, as we need to consider naming (on/off is ambiguous with
running an exit node, etc) as well as automatic exit node selection in
the future. For now the CLI commands are effectively developer debug
things to test the LocalAPI.

Updates tailscale/corp#18724

Change-Id: I9a32b00e3ffbf5b29bfdcad996a4296b5e37be7e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -833,6 +833,10 @@ func TestPrefFlagMapping(t *testing.T) {
 			// Handled by the tailscale share subcommand, we don't want a CLI
 			// flag for this.
 			continue
+		case "InternalExitNodePrior":
+			// Used internally by LocalBackend as part of exit node usage toggling.
+			// No CLI flag for this.
+			continue
 		}
 		t.Errorf("unexpected new ipn.Pref field %q is not handled by up.go (see addPrefFlagMapping and checkForAccidentalSettingReverts)", prefName)
 	}