Convert SMB and SMB2 code to use a common buffer handling structure

Convert SMB and SMB2 code to use a common buffer handling structure

This converts our SMB and SMB2 code to use a common structure "struct
request_bufinfo" for information on the buffer bounds of a packet,
alignment information and string handling. This allows us to use a
common backend for SMB and SMB2 code, while still using all the same
string and blob handling functions.

Up to now we had been passing a NULL req handle into these common
routines from the SMB2 side of the server, which meant that we failed
any operation which did a bounds checked string extraction (such as a
RenameInformation setinfo call, which is what Vista uses for renaming
files)

There is still some more work to be done on this - for example we can
now remove many of the SMB2 specific buffer handling functions that we
had, and use the SMB ones.