sftp: use the absolute path instead of the relative path
sftp: use the absolute path instead of the relative path

Before this change rclone used the relative path from the current
working directory.

It appears that WS FTP doesn't like this and the openssh sftp tool
also uses absolute paths which is a good reason for switching to
absolute paths.

This change reads the current working directory at startup and bases
all file requests from there.

See: https://forum.rclone.org/t/sftp-ssh-fx-failure-directory-not-found/17436

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone