Remove support for pre-authentication compression. Doing compression
early in the protocol probably seemed reasonable in the 1990s, but
today it's clearly a bad idea in terms of both cryptography (cf.
multiple compression oracle attacks in TLS) and attack surface.

Moreover, to support it across privilege-separation zlib needed
the assistance of a complex shared-memory manager that made the
required attack surface considerably larger.

Prompted by Guido Vranken pointing out a compiler-elided security
check in the shared memory manager found by Stack
(http://css.csail.mit.edu/stack/); ok deraadt@ markus@

NB. pre-auth authentication has been disabled by default in sshd
for >10 years.
destroy_sensitive_data(void)
{
	int i;

	for (i = 0; i < options.num_host_key_files; i++) {
		if (sensitive_data.host_keys[i]) {
			key_free(sensitive_data.host_keys[i]);
			sensitive_data.host_keys[i] = NULL;
		}
		if (sensitive_data.host_certificates[i]) {
			key_free(sensitive_data.host_certificates[i]);
			sensitive_data.host_certificates[i] = NULL;
		}
	}
}

destroy_sensitive_data(void)
{
	int i;

	for (i = 0; i < options.num_host_key_files; i++) {
		if (sensitive_data.host_keys[i]) {
			key_free(sensitive_data.host_keys[i]);
			sensitive_data.host_keys[i] = NULL;
		}
		if (sensitive_data.host_certificates[i]) {
			key_free(sensitive_data.host_certificates[i]);
			sensitive_data.host_certificates[i] = NULL;
		}
	}
}
