Two small problems with libpcap

Hi

I spotted two small problems with libpcap:

* Preables for received PP frames are missing, the dedected.org tools used to include the preamble as well. This can easily be fixed by adding a default preamble if the kernel doesn't report the preamble to the userspace. * The slottime wraps around at 2^12 and not at 2^11 as it used to wrap around with the dedected.org tools. I am not sure about the best solution here, one could also use the full 16 bits for the time which are available in the header.