This adds Apache support (taking advantage of the new APR capability)

for ldap rebind callback while chasing referrals. This allows direct
searches on LDAP servers (in particular MS Active Directory 2003+)
using referrals without the use of the global catalog.
This addresses PRs 26538, 40268, and 42557



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@614605 13f79535-47bb-0310-9956-ffa450edef68