mod_authnz_ldap: Allow using exec: callouts like SSLPassphraseDialog

for AuthLDAPBindPassword.

Backports: r1433478, r1467523, r1467792
Submitted by: druggeri
Reviewed by: minfrin. wrowe



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1497095 13f79535-47bb-0310-9956-ffa450edef68