ipn/ipn{local,server}: move "staying alive in server mode" from ipnserver to LocalBackend

Currently, we disconnect Tailscale and reset LocalBackend on Windows when the last LocalAPI client
disconnects, unless Unattended Mode is enabled for the current profile. And the implementation
is somewhat racy since the current profile could theoretically change after
(*ipnserver.Server).addActiveHTTPRequest checks (*LocalBackend).InServerMode() and before it calls
(*LocalBackend).SetCurrentUser(nil) (or, previously, (*LocalBackend).ResetForClientDisconnect).

Additionally, we might want to keep Tailscale running and connected while a user is logged in
rather than tying it to whether a LocalAPI client is connected (i.e., while the GUI is running),
even when Unattended Mode is disabled for a profile. This includes scenarios where the new
AlwaysOn mode is enabled, as well as when Tailscale is used on headless Windows editions,
such as Windows Server Core, where the GUI is not supported. It may also be desirable to switch
to the "background" profile when a user logs off from their device or implement other similar
features.

To facilitate these improvements, we move the logic from ipnserver.Server to ipnlocal.LocalBackend,
where it determines whether to keep Tailscale running when the current user disconnects.
We also update the logic that determines whether a connection should be allowed to better reflect
the fact that, currently, LocalAPI connections are not allowed unless:
 - the current UID is "", meaning that either we are not on a multi-user system or Tailscale is idle;
 - the LocalAPI client belongs to the current user (their UIDs are the same);
 - the LocalAPI client is Local System (special case; Local System is always allowed).
Whether Unattended Mode is enabled only affects the error message returned to the Local API client
when the connection is denied.

Updates #14823

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

ipn/ipnlocal/profiles.go

@@ -77,6 +77,48 @@ func (pm *profileManager) SetCurrentUserID(uid ipn.WindowsUserID) {
 	}
 }
 
+// SetCurrentUserAndProfile sets the current user ID and switches the specified
+// profile, if it is accessible to the user. If the profile does not exist,
+// or is not accessible, it switches to the user's default profile,
+// creating a new one if necessary.
+//
+// It is a shorthand for [profileManager.SetCurrentUserID] followed by
+// [profileManager.SwitchProfile], but it is more efficient as it switches
+// directly to the specified profile rather than switching to the user's
+// default profile first.
+//
+// As a special case, if the specified profile ID "", it creates a new
+// profile for the user and switches to it, unless the current profile
+// is already a new, empty profile owned by the user.
+//
+// It reports whether the call resulted in a profile switch.
+func (pm *profileManager) SetCurrentUserAndProfile(uid ipn.WindowsUserID, profileID ipn.ProfileID) (changed bool) {
+	pm.currentUserID = uid
+
+	if profileID == "" {
+		if pm.currentProfile.ID() == "" && pm.currentProfile.LocalUserID() == uid {
+			return false
+		}
+		pm.NewProfileForUser(uid)
+		return true
+	}
+
+	if profile, err := pm.ProfileByID(profileID); err == nil {
+		if pm.CurrentProfile().ID() == profileID {
+			return false
+		}
+		if err := pm.SwitchProfile(profile.ID()); err == nil {
+			return true
+		}
+	}
+
+	if err := pm.SwitchToDefaultProfile(); err != nil {
+		pm.logf("%q's default profile cannot be used; creating a new one: %v", uid, err)
+		pm.NewProfile()
+	}
+	return true
+}
+
 // DefaultUserProfileID returns [ipn.ProfileID] of the default (last used) profile for the specified user,
 // or an empty string if the specified user does not have a default profile.
 func (pm *profileManager) DefaultUserProfileID(uid ipn.WindowsUserID) ipn.ProfileID {