use `useradd` instead of `adduser` for install

as `adduser` is afaik either an alias for `useradd`, or sometimes a wild
wrapper around it, or even non-existant (i.e. archlinux), switch to the
more portable direct `useradd` call.

omitting `--group`, as `useradd(8)` states:

```
By default, a group will also be created for the new user (see [...])
```
Merge pull request #104 from zined/use_useradd_in_installation

use `useradd` instead of `adduser` for install
add systemd compatibility to install script
use proper systemd service type

as the sandstorm binary forks, reflect that behaviour in the systemd
unit configuration.