sftpman: add missing options

- `authType` is missing the `authentication-agent` enum value
- `mountDestPath` is missing completely

(cherry picked from commit 3351348827)

tests/modules/programs/sftpman/expected-mount3.json

@@ -3,6 +3,7 @@
   "beforeMount": "true",
   "host": "host3.example.com",
   "id": "mount3",
+  "mountDestPath": null,
   "mountOptions": [],
   "mountPoint": "/yet/another/path",
   "port": 22,