new plugin: openldap
openldap: note starttls in the doc
openldap: fix typo preventing PDU stats from getting collected
openldap: fix leak when reading from server fails
openldap: add threads monitoring
openldap: monitor every BDB-based database's cache
openldap: prepend function names with ldap_ to avoid confusion with apache plugin
openldap: fix double-free error on shutdown

This is already handled by ldap_unbind_ext_s() on line 739
openldap: coding style consistency
openldap: move config-handling functions after ldap_read_host()

This prevents the need for a dummy ldap_read_host() declaration.
openldap: revert change leading to use of uninitialised variable

this was preventing build with -Wall
openldap: use ldap_unbind_ext_s() to avoid memory leak

... as ldap_destroy() is only available in newer libldap versions.
openldap: terminate attrs array with NULL

... as mentioned in ldap_search_ext_s(3). This fixes a segfault when
building with -O2.
openldap: initialize LDAP library before collectd goes multi-threaded
openldap: add missing support for "Timeout" config option
openldap: prefer using ldap_err2string() over sstrerror()
openldap: coding style consistency
openldap: documentation improvements
openldap: initialize StartTLS option to it's default value
openldap: Remove custom string/bool/int config functions.

Roughly the equivalent of e4ab153f9, as this part of the code was copied
from the apache plugin.