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