www/freenginx-acme-module: Add new port

ngx_http_acme_module adds automatic ACME (RFC 8555) certificate
management to freenginx. This port builds the module against the
stable www/freenginx and installs it into libexec/freenginx.

While here, move the freenginx version into www/freenginx/version.mk
(matching www/nginx and www/nginx-devel) so the new port can include
it as the single source of truth.

Sponsored by: Netzkommune GmbH