r11595: added a helper layer to parse streams into individual packets. This is

r11595: added a helper layer to parse streams into individual packets. This is
something that Andrew Bartlett has been asking for for a while, and
when I started having to re-invent this packet parsing code yet again
for SMB2 I decided it was time to do it generically

you use it by providing a "is this a full packet yet?" helper function
to the packet_*() functions, which then handle all the logic of
partial packet buffering.

This also goes to great lengths to operate efficiently, minimising the
number of recv system calls.
(This used to be commit e6c47b954a6f09c53ea419800ce873295fcd0be9)