frontend.cpp: Use old code detection when DTV_ENUM_DELSYS ioctl fails

Using the latest linux-libc-headers the DTV_ENUM_DELSYS is defined
so it fails to properly identify supportsDeliverySystem.

Instead check if DTV_ENUM_DELSYS ioctl returns Invalid Argument (EINVAL)
and use old code detection.

Also print a debug message when unknown error occurs.