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>

ipn/prefs.go

@@ -790,6 +790,23 @@ type ProfileID string
 // tests.
 type WindowsUserID string
 
+// NetworkProfile is a subset of netmap.NetworkMap
+// that should be saved with each user profile.
+type NetworkProfile struct {
+	MagicDNSName string
+	DomainName   string
+}
+
+// RequiresBackfill returns whether this object does not have all the data
+// expected. This is because this struct is a later addition to LoginProfile and
+// this method can be checked to see if it's been backfilled to the current
+// expectation or not. Note that for now, it just checks if the struct is empty.
+// In the future, if we have new optional fields, this method can be changed to
+// do more explicit checks to return whether it's apt for a backfill or not.
+func (n NetworkProfile) RequiresBackfill() bool {
+	return n == NetworkProfile{}
+}
+
 // LoginProfile represents a single login profile as managed
 // by the ProfileManager.
 type LoginProfile struct {
@@ -804,13 +821,12 @@ type LoginProfile struct {
 	// It is filled in from the UserProfile.LoginName field.
 	Name string
 
-	// TailnetMagicDNSName is filled with the MagicDNS suffix for this
-	// profile's node (even if MagicDNS isn't necessarily in use).
-	// It will neither start nor end with a period.
+	// NetworkProfile is a subset of netmap.NetworkMap that we
+	// store to remember information about the tailnet that this
+	// profile was logged in with.
 	//
-	// TailnetMagicDNSName is only filled from 2023-09-09 forward,
-	// and will only get backfilled when a profile is the current profile.
-	TailnetMagicDNSName string
+	// This field was added on 2023-11-17.
+	NetworkProfile NetworkProfile
 
 	// Key is the StateKey under which the profile is stored.
 	// It is assigned once at profile creation time and never changes.