cmd/syncthing, lib/model: Handle rel/abs paths for config/protected paths (fixes #3183)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3995

cmd/syncthing/main.go

@@ -344,7 +344,13 @@ func main() {
 
 	if options.confDir != "" {
 		// Not set as default above because the string can be really long.
-		baseDirs["config"] = options.confDir
+		if !filepath.IsAbs(options.confDir) {
+			path, err := filepath.Abs(options.confDir)
+			if err != nil {
+				l.Fatalln(err)
+			}
+			baseDirs["config"] = path
+		}
 	}
 
 	if err := expandLocations(); err != nil {