Rewrite the LDAP server section of the handbook.

The Lightweight Directory Access Protocol (LDAP) section of the handbook
contains several errors (typos, wrong steps) and was in need of updates
to reflect current versions of the software. Rocky Hotas provided a patch
for review that fixes many of those bugs and extends the instructions in
the first part. The second part received a complete rewrite, providing
working example configuration files and steps to get the server running.

I made only a few cleanups and corrections to the patch to fit our
documentation guidelines. Most of the work was done by the submitter, big
thanks for his work and patience!

PR: 219142
Submitted by: rockyhotas@post.com
Reviewed by: remko, wollman, wblock, myself, others
Approved by: bcr
Differential Revision: https://reviews.freebsd.org/D10600