LDAP overhaul

LDAP overhaul

Allow replicate and update to be zero.
Break out logic into separate subs.
Do only one bind attempt depending on setting, instead of
necessarily failing first before trying auth_by_bind.

POD added for active directory and to document permutations of
behavior given different conditions.  Fixed mistaken debug lines
that called "print STDERR printf ...", i.e. printed the line to output
and "1" to the error log.  Added principal_name feature for generating
bind user from Koha userid.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>