Add SSL.close() and SSLClosed for orderly TLS shutdown

Pony's SSL wrapper had no way to send a TLS close_notify alert.
Tearing down a connection without one causes the peer to report a
protocol error instead of a clean closure, and some peers (notably
Go's net/http) treat that as a request failure.

SSL.close() calls SSL_shutdown to queue a close_notify in the output
BIO. SSLClosed is a new state that means the TLS session ended
cleanly — either the peer sent close_notify (detected by read) or
the consumer called close. The write side shuts down but the read
side stays open so the peer's remaining data and close_notify
response can still arrive.

SSL_ERROR_ZERO_RETURN from SSL_read and SSL_write now produces
SSLClosed instead of SSLError, giving consumers a way to distinguish
a clean peer closure from a protocol failure.

SSLConnection handles the new state: _do_shutdown sends the response
close_notify, flushes the encrypted bytes, and closes the TCP
connection.

Design: ponylang/lori#348

Author: SeanTAllen