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.