Bug 18578: Use subdirectory in /tmp for session storage during installation

Bug 18578: Use subdirectory in /tmp for session storage during installation

Apply the change from bug 15553 to InstallAuth.pm too.

Test plan:
[1] Remove all cgisess_* files from your /tmp directory.
[2] Remove directory /tmp/cgisess_koha_[your instance], if there.
[3] Run the webinstaller
    /cgi-bin/koha/installer/install.pl?step=1&op=updatestructure
[4] Check if you have cgisess_ files in /tmp/cgisess_koha_[your instance].

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>