lib/config, gui: Disallow some options in combination with "untrusted" (fixes #8920) (#8921)

This prevents combining untrusted with introducer and auto-accept, and
also verifies that folders shared with untrusted devices have passwords
at config loading time.

Co-authored-by: Simon Frei <freisim93@gmail.com>

lib/config/deviceconfiguration.go

@@ -34,6 +34,19 @@ func (cfg *DeviceConfiguration) prepare(sharedFolders []string) {
 	}
 
 	cfg.IgnoredFolders = sortedObservedFolderSlice(ignoredFolders)
+
+	// A device cannot be simultaneously untrusted and an introducer, nor
+	// auto accept folders.
+	if cfg.Untrusted {
+		if cfg.Introducer {
+			l.Warnf("Device %s (%s) is both untrusted and an introducer, removing introducer flag", cfg.DeviceID.Short(), cfg.Name)
+			cfg.Introducer = false
+		}
+		if cfg.AutoAcceptFolders {
+			l.Warnf("Device %s (%s) is both untrusted and auto-accepting folders, removing auto-accept flag", cfg.DeviceID.Short(), cfg.Name)
+			cfg.AutoAcceptFolders = false
+		}
+	}
 }
 
 func (cfg *DeviceConfiguration) IgnoredFolder(folder string) bool {