[Libguestfs] [PATCH nbdkit v3] ssh: Allow the remote file to be created

v1: https://listman.redhat.com/archives/libguestfs/2022-April/028680.html v2: https://listman.redhat.com/archives/libguestfs/2022-April/028682.html This version implements what I described here: https://listman.redhat.com/archives/libguestfs/2022-April/028691.html In brief, if you use create=true then the remote file is always created, existing content is removed if the file was already present. I think this is more predictable, and we can add further flags to refine the behaviour if we think that's necessary in future. Also this version uses setstat + SSH_FILEXFER_ATTR_PERMISSIONS to set the mode. I have checked and this is not affected by umask, so you get the exact mode that you request (at least, you do with OpenSSH). Also we try to clean up the remote file if creation fails, although it's best-effort only. Also use a lock to prevent parallel requests from trying to create the file twice. Rich.