Auto-loading can be toggled by setting the new autoload parameter
to FTDM_FALSE/FTDM_TRUE.

Update ftdm_span_create() and ftdm_api_execute() to use the new code.

NOTE: Auto-loading of missing I/O interfaces remains enabled in both cases,
      but I guess we should disable it for ftdm_api_execute().
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>