Add helper function to execute command w args and get one line of output....

Add helper function to execute command w args and get one line of output. Allow AuthLDAPBindPassword to have exec: argument like SSLPassPhraseDialog

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1433478 13f79535-47bb-0310-9956-ffa450edef68