make it possible to use regular (non EC) EDH ciphers. To make this
possible a Diffie-Hellman parameter must be passed to the openssl
library. There are a few options possible as described in the manuals at
[1] and [2]. Simplest approach is to generate a DH parameter using
openssl dhparam -C <lenght> and include the code into the application.
The lenght used for this commit is 2236 bits long, which is the longest
possible without risking backward incompatibilities with old systems as
stated in [1]. Newer systems should use ECDH anyway, so it makes sense
to keep this method as compatible with older system as possible.
Paramters longer than 2048 should still be secure enough at the time of
writing.
[1] https://wiki.openssl.org/index.php/Diffie-Hellman_parameters
[2] https://wiki.openssl.org/index.php/Manual:SSL_CTX_set_tmp_dh_callback(3)
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