added suppport for unexpected udp/138 packets

added suppport for unexpected udp/138 packets

I also fixed up the lookup_pdc_name() code so that it now works, even
with a NT server that insists on replying to udp/138.

The method I used to match packets was to use the mailslot string as a
datagram ID. The true dgm_id doesn't work as NT doesn't set it
correctly. uggh.

PS: Jeremy, I had to change your code quite a bit, are you sure this
worked with a Samba PDC?? The code looked broken, it got the offsets
wrong in the SMB portion of the packet and filled in the IP
incorrectly.
(This used to be commit 32f66f4ea63038cb4b3785bdf1762abdde076f5d)