Introduce the PassPhraseDialog 'pipe' mechanism.

  Introduce the PassPhraseDialog 'pipe' mechanism.

  This is the directive handling commit only, the mechanics patch will
  follow.  PassPhraseDialog "|/path/to/pipe" will use the bidirectional
  pipe to have a 'conversation', along the lines of the tty dialog with
  PassPhraseDialog 'builtin'.  This is entirely different than the 'exec'
  method, which simply runs once for each passphrase, and doesn't allow
  for failure/retries, and certainly doesn't offer any sensible 'dialog'.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93606 13f79535-47bb-0310-9956-ffa450edef68