Bug 18880: Fix authentication fallback for external authentications

Bug 18880: Fix authentication fallback for external authentications

A regression in commit cfc484b17 / bug #18314 breaks the local
authentication fallback for all external authentications like LDAP, CAS
and Shibboleth.

The regression itself is a logical error as "@return = (0)" is
considered to be "false" when checked with "unless" (line 1814). That's
wrong as "unless" tests the number of elements in a list.

This patch tries to simplify the logic by adding a $passwd_ok and
$check_internal_as_fallback flags to be more verbose and hopefully more
understandable.
The goal here is simply to restore back the same logic as before cfc484b17

Signed-off-by: Lee Jamison <ldjamison@marywood.edu>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>