r5108: the beginnings of a nbtd server for Samba4. Currently just displays

r5108: the beginnings of a nbtd server for Samba4. Currently just displays
the packets it receives, but it at least shows how the server
structure will work.

To implement it I extended the libcli/nbt/ library to allow for an
incoming packet handler to be registered. That allows the nbt client
library to be used for low level processing of the nbtd server packets.

Other changes:

 - made the socket library always set SO_REUSEADDR when binding to an
   interface, to ensure that restarts of a server don't have to wait
   for a couple of minutes.

 - made the nbt port configurable. Defaults to 137, but other ports
   will be useful for testing.
(This used to be commit 2fedca6adfd4df9e85cc86896dfa79630777a917)