This avoids the following warning:
Apr 19 10:53:48 xen systemd[1]: [/nix/store/...-unit-ddclient.service/ddclient.service:19] Unknown lvalue 'type' in section 'Service'
As `Type=simple` is the default in systemd, the assignment to the
service type can be simply dropped.
* rewrite to systemd.services
* disable forking to give systemd better control
* verifiably run as ddclient user
* expose ssl option
* unset default value for dyndns server
* rename option "web" to "use" to be consistent with ddclient docs
* add descriptions
* add types to options
* clean up formatting
