client: Enable method call return to exit or continue running

client: Enable method call return to exit or continue running

Enable a client method call return function to exit or continue running when
in non-interactive mode. When a connmanctl_dbus_method_return_func_t returns
-EINPROGRESS in non-interactive mode, the command line client is kept running
even after the method call has been processed.

The command line client needs to keep running after a successful Agent
registration in order to process Agent method calls. Also enabling tethering
in non-interactive mode needs the client executing until SSID, passphrase and
tethering mode have been set.