core: add better logging for autoconnect with a new log domain

Sometimes users report problems about profiles not autoconnecting and with the current logging it's difficult to understand where the problem is. Since all profiles must be evaluated when a device becomes available for autoconnection, adding by default a log message for each of them would be probably too verbose, even for TRACE level.

Add a new 'autoconnect' log domain that is disabled by default and is useful to debug autoconnect problems, and use it to log why profiles can't autoconnect.