SSL.File: Fix several issues in destroy().

destroy() still had several workarounds for the old backend approach
(with update_internal_state() et al), when attempting to close the
connection. Remove the workarounds and update the code to the new
backend approach.

Fixes issue with destroy() hanging in the local backend/throwing errors
due to non-existant local backend.

Also fixes a "Cannot access variables in destructed object." when
a read_callback destructs the SSL.File object.

Thanks to Tobias S. Josefowitz <tobij@tobij.de> for the report.

Fixes [LysLysKOM 21005505].
Ignore generated file wnotify_config.h.in.
Removed a few GMP tests.
SSL.ClientConnection: Don't send extensions in SSL 3.0.

SSL 3.0 doesn't have the concept of protocol extensions,
so don't send any.

In the ServerConnection case, we assume that a client
that has sent extensions will accept extensions regardless
of the negotiated protocol version.
SSL.Session: Survive the client asking for ECC without curves.
Testsuite: SSL 3.0 doesn't support ECC...