sftp: add missing interface check and fix About #3257
sftp: add missing interface check and fix About #3257

This bug was introduced as part of adding context to the backends and
slipped through the net because the About call did not have an
interface assertion in the sftp backend.

I checked there were no other missing interface assertions on all the
optional methods on all the backends.

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