res_config_ldap: Make memory allocation more consistent

The code in update_ldap() and update2_ldap() was using both Asterisk's
memory allocation routines as well as OpenLDAP's. I've changed it so
that everything that is passed to OpenLDAP's functions are allocated
with their routines.

Change-Id: Iafec9c1fd8ea49ccc496d6316769a6a426daa804