Some old stuff hanging around since the CIFS conference. Big cleanup of

Some old stuff hanging around since the CIFS conference.  Big cleanup of
rpcclient code.  Refactored cmd_* functions to move common mem_ctx and pipe
opening stuff up one level.  Moved rpcclient.h into rpcclient directory and
out of includes/smb.h
(This used to be commit a40facba9651f9fb1dcc9e143f92ca298a324312)