Re-allow backup set names containing "/"

Revert the prohibition because all releases up to now have allowed
"/", and so that's nothing new.  Disabling "/" in 0.26 would be a
regression, and one we're not ready to commit to -- in fact, we may
eventually do the opposite, and add comprehensive support for "/".

This reverts commit 52a98179490c6ccb990e221bbdeb511f1de59de4.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>