charon-nm: Allow configurable remote traffic selectors

charon-nm: Allow configurable remote traffic selectors

This change allows to customize the previously hard-coded remote traffic
selectors.

This does not actually write the newly added "remote-ts" configuration option
into NetworkManager's configuration file, but will use an existing value.
Exposing the config setting in the GUI could be done later if this is a
desired change.

Use case:  remote firewall appliance wrongly accepts the `0.0.0.0/0` TS but
does not actually route external traffic, leaving the user with a partially
working internet connection.

Closes strongswan/strongswan#173.