Bug 15816: Redirect back to correct page after login

Bug 15816: Redirect back to correct page after login

This uses a hacky but simple method to get the correct script name under
proxied packaged Plack.

Test plan:
  1) Log out of both the OPAC and staff side.
  2) Try to access a page that requires login (opac-reserve.pl is a
good one for the OPAC), then log in.
  3) You will be redirected back to mainpage.pl or opac-user.pl.
  4) Repeat above for both staff side and OPAC.
  5) Apply patch.
  6) Repeat steps 1-4; you should be redirected back to the original
     page you were on.
  7) Repeat the above for both a traditional CGI and kohadevbox/package
     Plack installation.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit bc707baf02c672fdd8f6d2095137694d7e400a97)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>