Use a consistent style of returning a new STREAM object from functions
that output data, rather than requiring an existing structure to be
passed in. This generally makes the memory management more straight
forward and allows us to do more proper bounds checking of everything.
This also adds some new STREAM macros to make it easier to manage
them without poking around in the internal structure.
This work is done by Nikita Krupenko which sent a patch to
rdesktop in 2010 for version 1.6.0. I have now ported it
over to trunk and it seems to work as expected.
