backport r1526168, r1527291, r1527294, r1527295 and r1527926 from trunk

Submitted by: kbrand
Reviewed by:

Streamline and improve ephemeral key handling:

- drop support for ephemeral RSA keys (only allowed/needed
  for export ciphers)

- drop pTmpKeys from the per-process SSLModConfigRec, and remove
  the temp key generation at startup (unnecessary for DHE/ECDHE)

- unconditionally disable null and export-grade ciphers by always
  prepending "!aNULL:!eNULL:!EXP:" to any cipher suite string

- do not configure per-connection SSL_tmp_*_callbacks, as it is
  sufficient to set them for the SSL_CTX

- set default curve for ECDHE at startup, obviating the need
  for a per-handshake callback, for the time being (and also
  configure SSL_OP_SINGLE_ECDH_USE, previously left out)

- increase minimum required OpenSSL version to 0.9.8a, as we
  now rely on the get_rfcX_prime_Y functions from <openssl/bn.h>

- 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

- 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