Bug 6679 - fix 3 perlcritic violations in C4/Auth.pm, and enabled warnings

Bug 6679 - fix 3 perlcritic violations in C4/Auth.pm, and enabled warnings

Subroutine prototypes used at line 561, column 1.  See page 194 of PBP.  (Severity: 5)

Bareword file handle opened at line 606, column 5.  See pages 202,204 of PBP.  (Severity: 5)

Two-argument "open" used at line 606, column 5.  See page 207 of PBP.  (Severity: 5)

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
This patch reintroduces 'use warnings' in C4/Auth.pm.
Keep attentive to new warning messages in your log

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>