Adds a new tls_external_cert_and_key config option for chatmail servers
that manage their own TLS certificates (e.g. via an external ACME client
or a load balancer).
A systemd path unit (tls-cert-reload.path) watches the certificate file
via inotify and automatically reloads dovecot and nginx when it changes.
Postfix reads certs per TLS handshake so needs no reload.
Also extracts openssl_selfsigned_args() so cert generation parameters
are shared between SelfSignedTlsDeployer and the e2e test.
