Shelving: remind user if old 1.10 shelves exist

If old shelves created by Subversion 1.10 exist in the WC, shelving should remind the user, for these reasons:

1. to help the user avoid losing their work (perhaps forgetting it exists and deleting the WC);
2. to help the user read the names of their old shelves (as the filenames are encoded);
3. to encourage the user to free up the ".svn/shelves" namespace

The reminder could be issued when "svn shelf-list" is used.

It would be sufficient to list the names of the old shelves, with a note explaining that the user can restore them using "svn patch .svn/shelves/FOO". It is not necessary to provide any further commands to manipulate them.

Note that a 1.10 shelf is in the form of a patch file and does not contain enough metadata to be converted easily, deterministically, automatically to a newer format.