Re-enable SSLv2 support to restore ABI.

Re-enable SSLv2 support to restore ABI.

Excerpt from CHANGES:

    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 and
    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.

Approved by:	re (marius, gjb), so (delphij)

git-svn-id: svn://svn.freebsd.org/base/stable/10@296371 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f