sftp: implement --sftp-copy-is-hardlink to server side copy as hardlink

If the server does not support hardlinks then it falls back to normal
copy.

See: https://forum.rclone.org/t/sftp-remote-server-side-copy/41867

fstest/testserver/init.d/TestSFTPOpenssh

@@ -16,6 +16,7 @@ start() {
     echo host=$(docker_ip)
     echo user=$USER
     echo pass=$(rclone obscure $PASS)
+    echo copy_is_hardlink=true
     echo _connect=$(docker_ip):22
 }