Minor refactoring: the fields `pktin' and `pktout' in the Ssh

Minor refactoring: the fields `pktin' and `pktout' in the Ssh
structure have been retired. Now all Packet structures are
dynamically allocated. Each rdpkt function allocates one, and it's
freed after being used; and the packet construction functions
allocate them too, and they're freed by the send functions.

`pktin' and `pktout' were ugly. They were _morally_ still global
variables; even though they were replicated per SSH session to
comply with the Mac no-globals requirement, they weren't really in
the _spirit_ of `dynamically allocate your data'.

As a side effect of this change, the `pktout_blanks' and
`pktout_nblanks' fields in the Ssh structure have been moved into
the Packet structure.

git-svn-id: svn://svn.tartarus.org/sgt/putty@4898 cda61777-01e9-0310-a592-d414129be87e