reuse existing private key if possible for all SSLPassPhraseDialog

types, not just builtin.  on win32 for example, a pipe dialog might
allocate a wintty for prompting, which results in 4 prompts at
startup, 2 for each child and 2 within each when httpd "restarts
itself".
update comments on this and wrap them a bit.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk/modules/ssl@93679 13f79535-47bb-0310-9956-ffa450edef68