BUG/MEDIUM: ssl: Fix a memory leak in DHE key exchange

OpenSSL does not free the DH * value returned by the callback specified with SSL_CTX_set_tmp_dh_callback(),
leading to a memory leak for SSL/TLS connections using Diffie Hellman Ephemeral key exchange.
This patch fixes the leak by allocating the DH * structs holding the DH parameters once, at configuration time.

Note: this fix must be backported to 1.5.
(cherry picked from commit 8de5415b85512da871d58d1e9a0a33bd67f3b570)