mod_authnz_ldap: Allow using exec: callouts like SSLPassphraseDialog

mod_authnz_ldap: Allow using exec: callouts like SSLPassphraseDialog
for AuthLDAPBindPassword.

trunk patch: http://svn.apache.org/viewvc?view=revision&revision=1433478
             http://svn.apache.org/viewvc?view=revision&revision=1467523
             http://svn.apache.org/viewvc?view=revision&revision=1467792
2.4.x patch: http://people.apache.org/~druggeri/patches/AuthLDAPBindPasswordExec-2.4.patch
             (20130119 - updated to include minor mmn bump)
             (20130412 - updated to not use static var - thx, wrowe)

Submitted by: druggeri
Reviewed by: jim, minfrin

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1476694 13f79535-47bb-0310-9956-ffa450edef68