Bug 28409: Comprehensively validate category in opac-shelves.pl

Bug 28409: Comprehensively validate category in opac-shelves.pl

Default to a category of 1 (ie Private). Only allow input of 1
or 2 (ie Public)

== Test plan ==
1. Go to http://localhost:8080/cgi-bin/koha/opac-shelves.pl?category=function(){window.location.href%20=%20%27https://git.koha-community.org/stats/koha-master/authors.html%27}()
2. Note that you are redirected to another website
3. Go to http://localhost:8080/cgi-bin/koha/opac-shelves.pl?op=add&shelfname=foo&category=9
4. Note that you can't see this list in the Lists (but it has been added to the database)
5. Apply the patch & restart services
6. Go to http://localhost:8080/cgi-bin/koha/opac-shelves.pl?category=function(){window.location.href%20=%20%27https://git.koha-community.org/stats/koha-master/authors.html%27}()
7. Note that you are not redirected to another website
8. Go to http://localhost:8080/cgi-bin/koha/opac-shelves.pl?op=add&shelfname=bar&category=9
9. Note that "bar" has been added as a Private list

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>