tls_dhparam take prime identifiers. Also unbreak combination of
OpenSSL+DH_params+TLSSNI.
+PP/39 Disable SSLv2 by default in OpenSSL support.
+
+PP/40 Lower default size of EXIM_CLIENT_DH_MIN_BITS constant (used only by
+ GnuTLS at this time) from 1024 to 512. Cautious folk can override
+ in Local/Makefile.
+
Exim version 4.77
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