systemd units: add template unit for different prefix

[shirok1]

With this feature merged, user can easily run OpenResty on different prefix with systemd

$ systemd-escape --template=openresty@.service -p /your/prefix
openresty@your-prefix.service

$ sudo systemctl status openresty@your-prefix.service
● openresty@your-prefix.service - The OpenResty Application Platform (/your/prefix)
....
    Process: xxxx ExecReload=/usr/local/openresty/nginx/sbin/nginx -p /your/prefix/ -g daemon on; master_process on; >
....

Looking for help on:

• Is new unit correctly included in deb builds?
• Should post, preun, postun hooks be changed on rpm builds?