Some operations are clearly asynchronous (e.g. monitoring modem state), while
others are clearly synchronous (e.g. printing modem info). So just allow option
groups to force the operation to be async or sync based on the detected action.
The original command line interface was written based on a hand-made libmm. This
commit ports the Manager interface handling to the new gdbus-codegen-based
libmm-glib.
