added the unexpected packet database (unexpected.tdb)

added the unexpected packet database (unexpected.tdb)
this means "nmblookup -S" now always works, even with broken servers
the database stores all unexpected replies and these can be accessed
by any client.
while doing this I cleaned up a couple of functions, and put in place
a better trn_id generator. in most places the code got quite a bit
simpler due to the addition of simple helper functions.

I haven't yet put the code in to take advantage of this for pdc
replies - that will be next. Jeremys pdc finding code will then work :)