always reusing existing private key for given vhost on restarts if key

is encrypted and mtime stamp has not changed.  this prevents getting
prompted twice for passphrase on windows and elsewhere when server is
started with -DNO_DETACH.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93620 13f79535-47bb-0310-9956-ffa450edef68