transp,tls: add TLS client verification

[maximilianfridrich]

Per default, TLS client verification is disabled.

[alfredh]

Perhaps move the bool to the API so that the feature can be turn on and off at any time ?

[maximilianfridrich]

I am unsure where I would move it. We have to call SSL_set_verify on the SSL object. Ideally, shortly after initializing it. For incoming connections, this happens in the tcp_connect_handler in transp.c and in this context we don't have much information except for the sip_transport which contains the tls struct.

This closely mirrors the approach of verify_server.

Maybe I misunderstood, could you specify where exactly I could move the bool? Or should I add tls_disable_verify_client to be able to disable the feature?

[sreimers]

I think @alfredh is looking for something like this:

void tls_enable_verify_client(struct tls *tls, bool enable) {
	if (!tls)
		return;

	tls->verify_client = enable;
}

[maximilianfridrich]

Ooh, okay. Done.