Improve ephemeral key handling (companion to r1526168):

- allow to configure custom DHE or ECDHE parameters via the
  SSLCertificateFile directive, and adapt its documentation
  accordingly (addresses PR 49559)

- add standardized DH parameters from RFCs 2409 and 3526,
  use them based on the length of the certificate's RSA/DSA key,
  and add a FAQ entry for clients which limit DH support
  to 1024 bits (such as Java 7 and earlier)

- move ssl_dh_GetParamFromFile() from ssl_engine_dh.c to
  ssl_util_ssl.c, and add ssl_ec_GetParamFromFile()

- drop ssl_engine_dh.c from mod_ssl

For the standardized DH parameters, OpenSSL version 0.9.8a
or later is required, which was therefore made a new minimum
requirement in r1527294.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1527295 13f79535-47bb-0310-9956-ffa450edef68