Add support for TLS-SRP (Secure Remote Password key exchange

for TLS, RFC 5054).

PR: 51075
Submitted by: Quinn Slack <sqs cs stanford edu>, Christophe Renou,
              Peter Sylvester


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1347980 13f79535-47bb-0310-9956-ffa450edef68