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

for TLS, RFC 5054).
Including some improvements as suggested by Kaspar

PR: 51075
Submitted by: Quinn Slack <sqs cs stanford edu>, Christophe Renou,
              Peter Sylvester
Backported by: sf
Reviewed by: sf, minfrin, rjung

Backports of r1347980 and r1348653 form trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1420057 13f79535-47bb-0310-9956-ffa450edef68