Problem reported by Dennis Wydra in
<http://lists.gnu.org/archive/html/bug-tar/2010-11/msg00082.html>.
* scripts/backup.in: Accept "-l LEVEL". Be more systematic about
backslashes inside ``; it shouldn't matter for modern shells but
it might matter for older ones.
* scripts/restore.in: Likewise.
* scripts/backup.in: Adjust implementation of -t/--time to match
the new implementation of -l/--level.