Use new limits@openssh.com protocol extension to let the client select

Use new limits@openssh.com protocol extension to let the client select
good limits based on what the server supports. Split the download and
upload buffer sizes to allow them to be chosen independently.

In practice (and assuming upgraded sftp/sftp-server at each end), this
increases the download buffer 32->64KiB and the upload buffer
32->255KiB.

Patches from Mike Frysinger; ok dtucker@