Increase minimum required OpenSSL version to 0.9.8a (in preparation

Increase minimum required OpenSSL version to 0.9.8a (in preparation
for the next mod_ssl commit, which will rely on the get_rfcX_prime_Y
functions added in that release):

- remove obsolete #defines / macros

- in ssl_private.h, regroup definitions based on whether
  they depend on TLS extension support or not

- for ECC and SRP support, set HAVE_X and change the rather awkward
  #ifndef OPENSSL_NO_X lines accordingly

For the discussion prior to taking this step, see
https://mail-archives.apache.org/mod_mbox/httpd-dev/201309.mbox/%3C524275C7.9060408%40velox.ch%3E

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1527294 13f79535-47bb-0310-9956-ffa450edef68