Make tls_close() more robust - do not rely on a close notify being received

Make tls_close() more robust - do not rely on a close notify being received
from the other side and only return TLS_READ_AGAIN/TLS_WRITE_AGAIN if we
failed to send a close notify on a non-blocking socket.

Otherwise be more forceful and always shutdown/close the socket regardless
of other failures. Also do not consider ENOTCONN or ECONNRESET to be a
shutdown failure, since there are various situations where this can occur.

ok doug@ guenther@