Bug 5131 :restrict use of sort_by value to allowed values

Bug 5131 :restrict use of sort_by value to allowed values

The user input for sort_by value was used without care, resulting the possibility for user to set any Template Variable to 1.

This patch restrict the values to sort field.
The list of allowd_sortby was taken from 'includes/resort_form.inc'.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
(cherry picked from commit 649573ad244b95e1b90df7d0925f31f735ae50f7)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>