resolve: several follow-ups for resolving service via varlink

resolve: several follow-ups for resolving service via varlink

Follow-ups for e1634bb8321c5534a8e5d16b474c7e9d43ef3baa.

- Allow to call the method without "name" and "type".
- Allow to specify SD_RESOLVE_NO_TXT and SD_RESOLVE_NO_ADDRESS.
- Allow to provide multiple services, and fix memory leak.
- Rearrange the return value format.
- Encode TXT field with octescape() to make the field matches with the
  io.systemd.Resolve.Monitor interface.

Fixes #31371.