ipn/ipnlocal,cmd/tailscale: persist tailnet name in user profile

This PR starts to persist the NetMap tailnet name in SetPrefs so that tailscaled
clients can use this value to disambiguate fast user switching from one tailnet
to another that are under the same exact login. We will also try to backfill
this information during backend starts and profile switches so that users don't
have to re-authenticate their profile. The first client to use this new
information is the CLI in 'tailscale switch -list' which now uses text/tabwriter
to display the ID, Tailnet, and Account. Since account names are ambiguous, we
allow the user to pass 'tailscale switch ID' to specify the exact tailnet they
want to switch to.

Updates #9286

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

types/netmap/netmap.go

@@ -177,6 +177,16 @@ func (nm *NetworkMap) MagicDNSSuffix() string {
 	return MagicDNSSuffixOfNodeName(nm.Name)
 }
 
+// DomainName returns the name of the NetworkMap's
+// current tailnet. If the map is nil, it returns
+// an empty string.
+func (nm *NetworkMap) DomainName() string {
+	if nm == nil {
+		return ""
+	}
+	return nm.Domain
+}
+
 // SelfCapabilities returns SelfNode.Capabilities if nm and nm.SelfNode are
 // non-nil. This is a method so we can use it in envknob/logknob without a
 // circular dependency.