No multiplexing by default, unless required by the setup

For QMI and MBIM modems, only enable multiplexing support if explicitly requested during a connection attempt (with multiplex=required). In other words, if not explicitly given, the new default is to not request multiplexing (multiplex=none). Before these changes, the default if none explicitly given was to enable multiplexing if supported (multiplex=requested).

For QMI/IPA based setups, the default is to always enable multiplexing, though, as there is no other way to setup the connection.

This change makes multiplexing an optional feature, which is how it's going to be released in the next MM 1.18 release, in order to give time to users of MM to properly test and support the multiplexed connections before they're made the default in a future release.