SSL.Session: Fixed some warnings.
Testsuite: Make sure that there are valid certs for old TLS.

Old versions of TLS (before TLS 1.2) can't use SHA256 certs
for most suites as the suites use SHA1 (or even MD5) which
is too weak for SHA256 certs.

This problem was introduced by the GnuTLS interop fix.
SSL.File: Move write_callback setting to queue_write().

The stream write_callback is now set (to ssl_write_callback) whenever
the write_buffer isn't empty, and cleared when it is.

This reduces the complexity and cleans up the semantics of where
and when the stream write_callback is changed.