Add user scope for UnitInstaller

[icanhazstring]

Currently the UnitInstaller does not work with user scope.
The $installPath is fix for system scope installations.

This should work similar to switching the scope to find a unit.

$systemCtl->user()->install($template); // default: ~/.config/systemd/user/
$systemCtl->system()->install($template); // default: /etc/systemd/system/

user home should be retrieved automatically.
http://php.net/manual/en/function.posix-getpwuid.php

To resolve the paths we should use the currently empty PathResolver