signaldctl: init at 0.6.1

signaldctl is a tool used to administer the signald service (already
available in nixpkgs). after enabling the signald service as below:

```nix
{
  services.signald.enable = true;
}
```

then a user can use signaldctl to introspect signald's state.
the communication happens over `/run/signald/signald.sock`, so either
configure signald to expose that with desired permissions, or run
signaldctl as the signald user:
```
$ sudo -u signald -g signald signaldctl account list
┌──────────────┬──────────────────────────────────────┬───────────┐
│ PHONE NUMBER │ UUID                                 │ DEVICE ID │
├──────────────┼──────────────────────────────────────┼───────────┤
│ +1XXXXXXXXXX │ XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX │         1 │
└──────────────┴──────────────────────────────────────┴───────────┘
```

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