[Minor] Allow more sane flags and ids syntax when register symbols

[Minor] Allow more sane flags and ids syntax when register symbols

For example, flags could be defined as:

```lua
rspamd_config.register_symbol{
    flags = ['ignore_passthrough', 'nice']
  }
```

instead of (compatibility is still there):
```lua
rspamd_config.register_symbol{
    flags = 'ignore_passthrough,nice'
  }
```