Bug 29915: Changes for get_session and check_cookie_auth

Bug 29915: Changes for get_session and check_cookie_auth

If we look for an existing session, do not create a new one.
Found a bug in the unset_userenv calls. For this moment
changing the calls in Auth here. Later fix goes to bug
29954.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>