some CI envs don't like `listen on localhost' but tolerate INADDR_ANY or IN6ADDR_ANY_INIT.

now it resembles less oconfigure and more the configure scripts I'm using in my recent projects. I'd argue it's more easy to use it.

Incorporate the OpenSMTPD' privsep crypto engine. The idea behind it is to never load the certificate' private keys in a networked process, instead they are loaded in a separate process (the `crypto' one) which signs payloads on the behalf of the server processes. This way, we greatly reduce the risk of leaking the certificate' private key should the server process be compromised. This currently compiles only on LibreSSL (portable fix is in the way).

it's better than the previous Makefile.depend approach since this automatically adapts to the included headers without requiring manual intervention to regen the list.