First phase of SFTP re-engineering. Each base-level fxp_* function

First phase of SFTP re-engineering. Each base-level fxp_* function
has been split into a send half and a receive half, so that callers
can set several requests in motion at a time and deal with the
responses in whatever order they arrive.

git-svn-id: svn://svn.tartarus.org/sgt/putty@3318 cda61777-01e9-0310-a592-d414129be87e