RFC 6176 compliance

RFC 6176 compliance

... prohibits use of SSLv2.
https://tools.ietf.org/html/rfc6176

Remove the documentation and support for configuring Squid with
SSLv2-only.

Explicitly enable the SSL_NO_SSLv2 option when provided by the library
to prevent implicit fallback.

Remove support for ssloptions= values which are for SSLv2-specific bugs.

Due to the way they are implemented with atoi() sslversion=N
configuration will still accept the values for SSLv2-only. But the
context creation will now unconditionally produce "SSLv2 not
supported" errors if the now undocumented values are attempted.