Change how netctl-auto looks for active interfaces

This fixes a bug where non-root users could not use netctl-auto to enable or disable individual profiles any more.

In order to get a list of interfaces for which the netctl-auto service is active, the netctl-auto utility currently looks for wpa_actiond pid files in `/run/network`.
However, since 8a41497 this will fail when executed by a non-root user, even though the user is in the `wheel` group and is actually allowed to control `wpa_supplicant` (and therefore also has permission to use netctl-auto).

This patch searches for running netctl-auto services in systemd in order to get a list of active interfaces.