MEDIUM: ssl: add the possibility to use a global DH parameters file

MEDIUM: ssl: add the possibility to use a global DH parameters file

This patch adds the ssl-dh-param-file global setting. It sets the
default DH parameters that will be used during the SSL/TLS handshake when
ephemeral Diffie-Hellman (DHE) key exchange is used, for all "bind" lines
which do not explicitely define theirs.