Non-ECC DHE ciphers in the `cipher_list` attribute of `ApiListener` (the
default value includes these) had no effect as no DH parameters were available
and therefore the server wouldn't offer these ciphers. OpenSSL provides
built-in DH parameters starting from version 1.1.0, however, these have to be
enables explicitly using the `SSL_CTX_set_dh_auto()` function. This commit does
so and thereby makes it possible to establish a connection to an Icinga 2
server using a DHE cipher.
a2e05f89e8