resolved: detect and warn other running LLMNR stack

Previously, `SO_REUSEADDR` is set before `bind`-ing socket, Thus,
even if another LLMNR stack is running, `bind` always success and
we cannot detect the other stack. By this commit, we first try to
`bind` without `SO_REUSEADDR`, and if it fails, show warning and
retry with `SO_REUSEADDR`.