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

Thanks to ulrich for the preliminary patch.

Closes Redmine#4049.

Run rsync command-line through a shell, so that single-quotes around excludes are interpreted (Closes: #677410)

When --exclude 'something' is passed directly to rsync without going through
a shell, rsync tries to match a file whose name contains single quotes, which is
definitely not what we want it to do: exclude arguments protected with single
quotes are meant to be passed to a shell.

The latter has a robust history wrt. dealing with excludes / glob / quoting
etc., let's use that instead of the shiny new sed-based way.

Hopefully this fixes http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=656968#31
and future incarnations of this class of bugs.

Using 'warning' and 'return' instead of a fatal at rotate_long if
backuproot doesn't exist. Using a warning ensures that the backup
action can process the other includes and not just exit.

Quoting $exclude and $excludes and avoiding a for loop on $exclude to not expand wildcards in beforehand (upstream #3882)

duplicity 0.6.17 uses paramiko backend.
Thus, it does not support limiting bandwidth anymore.
Let's use trickle to support bandwidth shaping anyway.

... and warn about unsupported options.

Locking its tables require privileges the 'debian-sys-maint'@'localhost' user is
generally not granted.