ipn/ipn{local,server}: extract logic that determines the "best" Tailscale profile to use

In this PR, we further refactor LocalBackend and Unattended Mode to extract the logic that determines
which profile should be used at the time of the check, such as when a LocalAPI client connects or disconnects.
We then update (*LocalBackend).switchProfileLockedOnEntry to to switch to the profile returned by
(*LocalBackend).resolveBestProfileLocked() rather than to the caller-specified specified profile, and rename it
to switchToBestProfileLockedOnEntry.

This is done in preparation for updating (*LocalBackend).getBackgroundProfileIDLocked to support Always-On
mode by determining which profile to use based on which users, if any, are currently logged in and have an active
foreground desktop session.

Updates #14823
Updates tailscale/corp#26247

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnserver/actor.go

@@ -32,6 +32,7 @@ type actor struct {
 	ci   *ipnauth.ConnIdentity
 
 	clientID ipnauth.ClientID
+	userID   ipn.WindowsUserID // cached Windows user ID of the connected client process.
 	// accessOverrideReason specifies the reason for overriding certain access restrictions,
 	// such as permitting a user to disconnect when the always-on mode is enabled,
 	// provided that such justification is allowed by the policy.
@@ -59,7 +60,14 @@ func newActor(logf logger.Logf, c net.Conn) (*actor, error) {
 		// connectivity on domain-joined devices and/or be slow.
 		clientID = ipnauth.ClientIDFrom(pid)
 	}
-	return &actor{logf: logf, ci: ci, clientID: clientID, isLocalSystem: connIsLocalSystem(ci)}, nil
+	return &actor{
+			logf:          logf,
+			ci:            ci,
+			clientID:      clientID,
+			userID:        ci.WindowsUserID(),
+			isLocalSystem: connIsLocalSystem(ci),
+		},
+		nil
 }
 
 // actorWithAccessOverride returns a new actor that carries the specified
@@ -106,7 +114,7 @@ func (a *actor) IsLocalAdmin(operatorUID string) bool {
 
 // UserID implements [ipnauth.Actor].
 func (a *actor) UserID() ipn.WindowsUserID {
-	return a.ci.WindowsUserID()
+	return a.userID
 }
 
 func (a *actor) pid() int {