First cut at speeding up SFTP. Generic download-management code in

sftp.c, and psftp.c now uses that instead of going it alone. Should
in principle be easily installed in PSCP as well, but I haven't done
it yet; also it only handles downloads, not uploads, and finally it
doesn't yet properly calculate the correct number of parallel
requests to queue. Still, it's a start, and in my own tests it
seemed to perform as expected (download speed suddenly became
roughly what you'd expect from the available bandwidth, and
decreased by roughly the expected number of round-trip times).

git-svn-id: svn://svn.tartarus.org/sgt/putty@3468 cda61777-01e9-0310-a592-d414129be87e