Reduce the number of round-trips involved in opening an SSH-2 session

Reduce the number of round-trips involved in opening an SSH-2 session
by sending most of the initial SSH_MSG_CHANNEL_REQUEST messages before
waiting for any replies.  The initial version of this code was a clever
thing with a two-pass loop, but that got hairy so I went for the simpler
approach of separating the request and reply code and having flags to
keep track of which requests have been sent.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9599 cda61777-01e9-0310-a592-d414129be87e