Bug 2940 - private shelf (list) display error on 2nd and subsequent pages.

Bug 2940 - private shelf (list) display error on 2nd and subsequent pages.

The before/after URL for the 2nd page of list elements:
http://atz.dev.kohalibrary.com/cgi-bin/koha/opac-shelves.pl?display=privateshelves?viewshelf=120&itemoff=2
http://atz.dev.kohalibrary.com/cgi-bin/koha/opac-shelves.pl?viewshelf=120&display=privateshelves&itemoff=2

The change in query string arguments is incidental, the prevention of multiple "?" is essential.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>