Add more support surrounding OpenSSL config stuff++..

Add more support surrounding OpenSSL config stuff++..

Some tweaks for OpenSSL:

. Instead of adding more and more specific variable( chain)(s)
  introduce a new *ssl-config-pairs* chain that offers direct
  interaction with SSL_CONF_cmd(3) if that is available, therefore
  offering access to _all_ possible commands of said command, but
  otherwise the well-known directives are supported via a builtin
  parser.

  Obsolete all variables superseded by this new mechanism:
  ssl-cert, ssl-cipher-list, ssl-curves, ssl-key, ssl-protocol,
  as well as ssl-method which was superseded by ssl-protocol
  already.

. Support SSL_CTX_config(3).
  The new *ssl-config-module* can now be used to outsource the
  entire SSL/TLS configuration to a central configuration file.

  *ssl-config-file* is now filename transformed if it is set to
  anything but an empty string.

. Support more variable chains: *ssl-ca-dir*, *ssl-ca-file*,
  *ssl-ca-flags*, *ssl-ca-no-defaults* are now all chains.

. Add a new read-only *ssl-features* variable which announces
  availability of several options to the user.