archwiki: use a drop-in file for memcached@.service instead of an entirely custom unit

This allows to retain all default hardening options that memcached@.service has.

roles/archwiki/tasks/main.yml

@@ -87,13 +87,13 @@
   service: name=php-fpm@{{ archwiki_user }}.socket state=started enabled=true
 
 - name: create memcached.service.d drop-in directory
-  file: path=/etc/systemd/system/memcached.service.d state=directory owner=root group=root mode=0755
+  file: path=/etc/systemd/system/memcached@archwiki.service.d state=directory owner=root group=root mode=0755
 
 - name: install memcached.service drop-in
-  template: src="memcached.service.d-archwiki.conf.j2" dest="/etc/systemd/system/memcached.service.d/archwiki.conf" owner=root group=root mode=0644
+  template: src="memcached.service.d-archwiki.conf.j2" dest="/etc/systemd/system/memcached@archwiki.service.d/archwiki.conf" owner=root group=root mode=0644
 
 - name: start and enable memcached service
-  service: name=memcached.service state=started enabled=true daemon_reload=true
+  service: name=memcached@archwiki.service state=started enabled=true daemon_reload=true
 
 - name: install systemd services/timers
   template: src="{{ item }}.j2" dest="/etc/systemd/system/{{ item }}" owner=root group=root mode=0644