SSL.handshake: Support setting the minimum required version.

The SSL 3.0 protocol can now be disabled by setting min_version in
the context to SSL.Constants.PROTOCOL_TLS_1_0.

Backported API from Pike 8.0. Note that the constants differ
from Pike 8.0 and later, so use the provided symbolic values.
SSL.Constants: Make the PROTOCOL_* constants forward compatible.

As these values have a tendency to get stuck in configuration files
and the like, make sure that they are forward-compatible with Pike 8.0.
SSL: Fix "Invalid description".

Some alerts were missing from ALERT_description.
release number bumped to 896 by export.pike
release number bumped to 897 by export.pike
SSL: Added some more Autodoc.