duplicity: for local backups, check that the destination directory exists.

Thanks to ulrich for the preliminary patch.

Closes Redmine#4049.
fix quoting on srcconffile and destcconffile
stop attempting to sync to current_backup, because if we do this then we do this then we don't take advantage of the hardlinking that is done later
the 'fatal' here was causing backups to stop after they hit a user who has
trouble, causing all subsequent backups never to fire.
set autotools markers and default application paths
remove some debugging bits that snuck in
dsync: fix loop number of iterations, we want to try 3 times, and we are starting at 0
dsync: break out of the while loop using 'break' instead of setting the failedcount number, otherwise this will trigger the warning later