Change the format of the SSL_{CLIENT,SERVER}_{I,S}_DN variables

Change the format of the SSL_{CLIENT,SERVER}_{I,S}_DN variables
to be RFC 2253 compatible, convert non-ASCII characters to UTF8, and
escape other special characters with backslashes. The old format can
still be used with the LegacyDNStringFormat argument to SSLOptions.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1054323 13f79535-47bb-0310-9956-ffa450edef68