Minor IWD improvments related to secrets and the config file writing

A few changes to the new IWD config file generation feature and secret requests.

The one change that touches core code is the addition of the NM_SETTINGS_CONNECTION_UPDATE_REASON_UPDATE_NON_SECRET flag that, like NM_SETTINGS_CONNECTION_UPDATE_REASON_RESET_SYSTEM_SECRETS and NM_SETTINGS_CONNECTION_UPDATE_REASON_RESET_AGENT_SECRETS tells the NMSettingConnection's updated signal handlers what has changed in the connection.