Specter fails to communicate with a built-in Bitcoin Core node

I am trying internal Bitcoin Core node on Linux, encountered a bunch of problems with getting a correct status of it.
At the end of the setup there was an error: signal only works in main thread.
Node have started, but node settings in Specter show that it doesn't - so I can't stop it. If I try to start it I get an error:

```
File "cryptoadvance/specter/internal_node.py", line 132, in start
File "cryptoadvance/specter/process_controller/bitcoind_controller.py", line 74, in attach_to_proc_id
File "signal.py", line 47, in signal
ValueError: signal only works in main thread
```

On the screenshot you see that the node is up and syncing, but Specter shows that it's down.


@k9ert

Start with cryptoadvance/specter/internal_node.py and cryptoadvance/specter/process_controller/bitcoind_controller.py, following the traceback through start and attach_to_proc_id. Reproduce the built-in Bitcoin Core setup on Linux and inspect the signal handling path. Done means Specter reports the running node correctly and starting or stopping it no longer raises the main-thread signal error.

Written by the indexing model from the issue text.