sslutil: store OP_NO_SSL* constants in module scope
An upcoming patch will introduce a global SSLContext type so we
have a single function used to wrap sockets. Prepare for that by
introducing module level constants for disabling SSLv2 and SSLv3.
sslutil: better document state of security/ssl module
Pythons older than 2.7.9 are lacking the modern ssl module
and have horrible security. Let's document this explicitly.