transmission: retrieve boolean config opts using `config_get_bool`

The tranmission UCI config options

- `config_overwrite`
- `incomplete_dir_enabled`
- `watch_dir_enabled`

are all booleans, so we have to retrieve them using `config_get_bool` in order
to make sure they are properly interpreted in case the user sets them to a
keyword (`true`/`false`, `on`/`off` etc.) and not an integer (`0`/`1`).

Signed-off-by: Salim B <git@salim.space>