sftp: support dynamic --sftp-path-override

Before this change, rclone always expected --sftp-path-override to be
the absolute SSH path to remote:path/subpath which effectively made it
unusable for wrapped remotes (for example, when used with a crypt
remote, the user would need to provide the full decrypted path.)

After this change, the old behavior remains the default, but dynamic
paths are now also supported, if the user adds '@' as the first
character of --sftp-path-override. Rclone will ignore the '@' and
treat the rest of the string as the path to the SFTP remote's root.
Rclone will then add any relative subpaths automatically (including
unwrapping/decrypting remotes as necessary).

In other words, the path_override config parameter can now be used to
specify the difference between the SSH and SFTP paths. Once specified
in the config, it is no longer necessary to re-specify for each
command.

See: https://forum.rclone.org/t/sftp-path-override-breaks-on-wrapped-remotes/40025

backend/sftp/sftp.go

@@ -168,7 +168,19 @@ E.g. if shared folders can be found in directories representing volumes:
 
 E.g. if home directory can be found in a shared folder called "home":
 
-    rclone sync /home/local/directory remote:/home/directory --sftp-path-override /volume1/homes/USER/directory`,
+    rclone sync /home/local/directory remote:/home/directory --sftp-path-override /volume1/homes/USER/directory
+	
+To specify only the path to the SFTP remote's root, and allow rclone to add any relative subpaths automatically (including unwrapping/decrypting remotes as necessary), add the '@' character to the beginning of the path.
+
+E.g. the first example above could be rewritten as:
+
+	rclone sync /home/local/directory remote:/directory --sftp-path-override @/volume2
+	
+Note that when using this method with Synology "home" folders, the full "/homes/USER" path should be specified instead of "/home".
+
+E.g. the second example above should be rewritten as:
+
+	rclone sync /home/local/directory remote:/homes/USER/directory --sftp-path-override @/volume1`,
 			Advanced: true,
 		}, {
 			Name:     "set_modtime",
@@ -1763,6 +1775,9 @@ func (f *Fs) remotePath(remote string) string {
 func (f *Fs) remoteShellPath(remote string) string {
 	if f.opt.PathOverride != "" {
 		shellPath := path.Join(f.opt.PathOverride, remote)
+		if f.opt.PathOverride[0] == '@' {
+			shellPath = path.Join(strings.TrimPrefix(f.opt.PathOverride, "@"), f.absRoot, remote)
+		}
 		fs.Debugf(f, "Shell path redirected to %q with option path_override", shellPath)
 		return shellPath
 	}