v0.16: RequestTTY=yes for SSH sessions

* Added RequestTTY=yes to ssh sesssions.  Hoping this will prevent random lockups.
v0.17: Warn when archive directories cannot be deleted

* If an archive directory which should be empty could not be deleted backrest was throwing an error.  There's a good fix for that coming, but for the time being it has been changed to a warning so processing can continue.  This was impacting backups as sometimes the final archive file would not get pushed if the first archive file had been in a different directory (plus some bad luck).
Fixed up README