crypto: allow runtime opt in using SSLv2/SSLv3

This change disables SSLv2/SSLv3 use by default, and introduces a
command line flag to opt into using SSLv2/SSLv3.

SSLv2 and SSLv3 are considered unsafe, and should only be used in
situations where compatibility with other components is required and
they cannot be upgrade to support newer forms of TLS.