os: handle relative and abs paths in Executable()

src/os/executable_darwin.go

@@ -6,5 +6,14 @@ package os
 func runtime_executable_path() string
 
 func Executable() (string, error) {
-	return runtime_executable_path(), nil
+	p := runtime_executable_path()
+	if p != "" && p[0] == '/' {
+		// absolute path
+		return p, nil
+	}
+	cwd, err := Getwd()
+	if err != nil {
+		return "", err
+	}
+	return joinPath(cwd, p), nil
 }