Disable SSLv2 default build, default negotiation and weak ciphers.

SSLv2 is by default disabled at build-time.  Builds that are not
configured with "enable-ssl2" will not support SSLv2.  Even if
"enable-ssl2" is used, users who want to negotiate SSLv2 via the
version-flexible SSLv23_method() will need to explicitly call either
of:

    SSL_CTX_clear_options(ctx, SSL_OP_NO_SSLv2);
or
    SSL_clear_options(ssl, SSL_OP_NO_SSLv2);

as appropriate.  Even if either of those is used, or the application
explicitly uses the version-specific SSLv2_method() or its client
or server variants, SSLv2 ciphers vulnerable to exhaustive search
key recovery have been removed.  Specifically, the SSLv2 40-bit
EXPORT ciphers, and SSLv2 56-bit DES are no longer available.

Mitigation for CVE-2016-0800

Reviewed-by: Emilia Käsper <emilia@openssl.org>

ssl/s2_lib.c

@@ -156,6 +156,7 @@ OPENSSL_GLOBAL const SSL_CIPHER ssl2_ciphers[] = {
      128,
      },
 
+# if 0
 /* RC4_128_EXPORT40_WITH_MD5 */
     {
      1,
@@ -171,6 +172,7 @@ OPENSSL_GLOBAL const SSL_CIPHER ssl2_ciphers[] = {
      40,
      128,
      },
+# endif
 
 /* RC2_128_CBC_WITH_MD5 */
     {
@@ -188,6 +190,7 @@ OPENSSL_GLOBAL const SSL_CIPHER ssl2_ciphers[] = {
      128,
      },
 
+# if 0
 /* RC2_128_CBC_EXPORT40_WITH_MD5 */
     {
      1,
@@ -203,6 +206,7 @@ OPENSSL_GLOBAL const SSL_CIPHER ssl2_ciphers[] = {
      40,
      128,
      },
+# endif
 
 # ifndef OPENSSL_NO_IDEA
 /* IDEA_128_CBC_WITH_MD5 */
@@ -222,6 +226,7 @@ OPENSSL_GLOBAL const SSL_CIPHER ssl2_ciphers[] = {
      },
 # endif
 
+# if 0
 /* DES_64_CBC_WITH_MD5 */
     {
      1,
@@ -237,6 +242,7 @@ OPENSSL_GLOBAL const SSL_CIPHER ssl2_ciphers[] = {
      56,
      56,
      },
+# endif
 
 /* DES_192_EDE3_CBC_WITH_MD5 */
     {